I worked on a financial product in a startup. The billing and collection features were so closely tied to the product that it would be quite risky to outsource

I worked on a financial product in a startup. The billing and collection features were so closely tied to the product that it would be quite risky to outsource that when you're still trying to find out what your product is exactly. In fact, we outsourced part of the software for the late payments collection process to a reputable SaaS and the results were not great. My take is that if your needs are fairly standard, you can benefit from such service. Especially your time to market will be shorter. But you won't face any disaster should you decide to do it yourself. If you want to innovate how you bill people (most Fintechs do), you should develop in house.