How to choose the right integration solution for HR/Payroll

Having a hard time planning HR/Payroll integrations? We've got you covered.
Written by
Graham Grieve
Published on
Friday, 27 October, 2023

Today, customers demand modern software that integrates with their existing tools, and will often make purchasing decisions based on available integrations.

This is true especially in B2B and HR/Payroll.

The decision to build custom integrations in-house or invest in a unified API can be a daunting one.

We’ve put together the following guide to help you in that process, comparing the options, and which type of solutions are best for different situations.

Option 1: “Integrating” manually, via CSV.

One option, of course, is to not actually integrate at all.

The problem with this route is that it likely requires your customers and you sending sensitive employment data back and forth via CSVs or SFTP.

It’s time consuming, it’s costly, data entry errors are numerous, the security risk is significant, and customers desire a better UX.

All-in-all, this route may have gotten you to where you are now, but it won’t get you where you want to go.

Option 2: Building Integrations In-house

Building integrations in-house offers complete control over the development process, but it demands substantial time, resources, and attention.

Building in-house = tons of admin, lots of engineers, and in some cases, lengthy negotiations with HR or Payroll platforms.

When considering the expensive developer time to build and maintain integrations, and risks involved if anything goes wrong, you’re looking at potentially 6 digit expenses—per integration.

Integrations will disrupt your entire product roadmap unless you hire out an entire integration team, which of course is not cheap.

Meanwhile, it’s quite possible your competitors are using off-the-shelf products and using those integrations to acquire new customers.

In summary, building each integration is likely the least cost effective solution.

Option 3: API aggregators, AKA “Unified APIs”

In the last few years, a crop of API aggregators have emerged: Kombo, Merge, Mistho, Argyle, Pinwheel, Rollee, etc..  

Pinwheel, Argyle, Mistho, Rollee, and many others are employee-side APIs. This means they get data from the employee/consumer side of products. If you want data for an entire company, (likely if you’re B2B) these are not the APIs you’re looking for.

Other API aggregators, such as Merge and Kombo, are employer side, meaning they provide integrations to admin level data, which is more relevant as a B2B company.

Merge has the probably the most comprehensive suite of APIs out there. Merge is a single API to multiple systems across HR, ATS, Accounting, CRM, and Directory.

However, they are primarily US-focused when it comes to integrations.

Merge provides their unified API by aggregating any public API available into one unified schema. If you have a very standard use case of HR, ATS, Accounting, CRM, or Directory data that would be provided for in traditional API docs, Merge may be a good fit.

However, as a standard unified API, AKA an "API aggregator", Merge's integrations are only as good as the APIs of the systems they aggregate. These underlying systems a) may not have an API, b) may limit the endpoints you can access, and c) can limit the companies and use cases that use it, depending on the situation.

Let's look at each one of these individually: 

A) Not every system provides a public API, such as many UK and European HR/Payroll systems (i.e. Pento, Sage Payroll, Quickbooks UK, Brightpay Connect, etc.). If it doesn't have a public API, unified APIs like Merge have nothing to aggregate.

B) You may want your software to see the data your HR admin sees in their systems, but many systems that have APIs don't provide access to all the endpoints you may want. As a result, Merge's integration with that system is constrained by the same factor.

C) Third, many systems will have stipulations for the use cases of its API. In some situations, they may prevent write integrations because they may want to remain as the hub of the customer experience, and want to prevent disintermediation. Another example is these systems do not enable complete data migration for churning customers using their API because they don't want to facilitate that churn. And lastly, if your product (or product roadmap) is competing with one of theirs, they may even kick you off their API because they don't want to lose out on revenue opportunities. As an aggregator of these APIs, Merge is limited by all these same constraints.

This is not to say that Merge is not the right solution in some situations. If you have a standard use case, just need to get high level employee data, don't have any plans to expand your product offering in ways that may compete with these underlying systems' products, Merge could be a good solution for you due to its coverage.

However, if you compete with any of the underlying systems, need deeper integrations, or are solely focused on Europe, Merge might not be the right choice.

Kombo is essentially seeking to be Merge for Europe. They have a number of integrations across ATS and HRIS, across the European market.

Like Merge, Kombo is a Unified API, aggregating public APIs into one unified schema. And, as such, they have the same constraints as Merge when it comes to systems, data, and use cases.

All that being said, if you need standard integrations with HRIS or ATS in Europe, Kombo may be a good fit, since it'll save you significant time, effort, and money if you're considering building your own integrations.

However, if you need access to systems without a public API, specific endpoints, or certain use cases that go against the strategic and competitive interests of the underlying system, Kombo may not have what you’re looking for.

Option 4: Vertically-Integrated API

The recurring theme in the standard Unified APIs is they are good for standard integrations, and systems with publicly available APIs.

However, if you don't want any gaps in your integrations, if you want access to any endpoint, or if you have more complex use cases that may take user engagement away from the underlying provider, the standard unified API may fall short.

This is where a vertically-integrated API, such as Affix, can help.

Instead of simply aggregating publicly available APIs, vertically-integrated APIs build their own API into the underlying HR/Payroll system. Plaid, the company that powered the Fintech revolution, uses this method.

Any system, even those that don't have a public API, have an internal API (aka, a developer API, or First Party API), that connects the systems' back end to its front end. Vertically-Integrated APIs, like Affix, integrate directly with these internal APIs.

The result is that Affix can provide access to any cloud system, endpoint, and in any use case.

For example: many HR/Payroll systems in the UK and Europe simply lack public APIs altogether, meaning that API aggregators, like Merge and Kombo, can’t provide integrations into these systems.

A Vertically Integrated API, however, provides a single integration into multiple systems – even those that lack public APIs.

And, as the name suggests, it provides that same unified schema like Merge and Kombo above that prevents you from building every integration yourself.

Affix is the Vertically Integrated API for HR/Payroll, and the pioneer in this space.

If you need access to HR/Payroll systems that lack a public API, Affix is a good choice for you. As a Vertically Integrated API, Affix can give you access to any cloud HR/Payroll system, not just the public ones that Merge and Kombo aggregate.

Secondly, with Affix, you can access any endpoint you need, even if it's not available in the system's public API documentation. If an HR/Payroll admin can see the data in their portal, that is data Affix can provide an endpoint for.

Third, with Affix, you can build integrations for any use case. Need write integrations? Affix can enable that. Want to help new customers migrate data from their old system via API? Affix can enable that as well. And, with Affix, you will never worry about your integration being kicked off, even if your product competes with revenue opportunities of the underlying system.

Lastly, Affix can provide a more bespoke and configurable experience: Affix provides detailed webhooks, gives you access to any data that your customers have access to and want to provide to you, and provides custom scopes, allowing you to only pull the data you need from customers. This last piece can prevent you from pulling data that you don't need, which can spook your customer.

To sum-up, Affix, as a Vertically Integrated API, is the best choice if you need access to systems without a public API, need deeper integrations, or want an API integration at every touch point between customers' systems.

Is a Vertically-Integrated API secure?


Affix uses JWTs as our API key, which is a more secure way for enabling access. Additionally, the Affix tech stack is serverless, so it doesn’t store any customer HR/Payroll data.

Can you use a Unified API and a Vertically Integrated API together?

Absolutely. It’s very possible you need integrations with a number of systems, some of which have public APIs, and some which do not.

Maybe you’re already using a Unified API, but doing manual integrations with CSVs for systems that lack a public API and Merge or Kombo can’t provide an integration for.

Affix covers these integrations, and uses the same endpoints as both Merge and Kombo. As a result, it’s easy to use both Affix alongside Merge and Kombo.

What’s the difference between a Unified API and a Vertically Integrated API for our customers?

Vertically Integrated APIs and Unified APIs work in the same way for your customers. Customers simply enter their credentials, and that data flows into your system seamlessly.

The UX for your customers is largely the same between a Unified API and a Vertically Integrated API.  


Overall, understanding your specific business needs is key.

While building in-house solutions offer control, the complexity and maintenance costs make it less viable.

Unified APIs, such as Kombo and Merge, provide a streamlined integration process, but are best for standard integrations in certain fields.

For those who need access to systems without public APIs, deep integrations, or use cases not available elsewhere, a Vertically Integrated API like Affix is the best solution.

For more information, feel free to book a consultation call with Affix.

Book a consulting call— it's free.
Need some guidance? Our team is available to hear about the challenges you face, and guide you in making the best integration decisions.
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