Payment processing is one area is that just a complete nonstarter to homebrew on your own. It's fucking awful. There's no choice. Bank's still require you to transfer them files via ftp to make transactions.
And god forbid the legal, and technical mess when you want to accept foreign currency, or even multiple CC providers.
The OEM required us to stand-up the servers so they could send us the data.
We also still use FTP for some vendors (we host).
"Homebrew on your own" is more or less what Stripe's API felt like though; the perspective I'm coming from is that I run a little one-person SaaS company with a simple and fairly standard subscription model, and I was surprised by the amount of "plumbing" you need to do for this.
I used them for a side project. Once you find the right documentation it's straightforward to integrate, but finding it took 3 days and 2 support requests.