At work we have a service built on powerdns-backend (http). The biggest pain there was lack of docs. There's circumstances where powerdns would make multiple requests to the backend app server, and figuring out how to answer its first direct queries in a way to satisfy it on the first query took some trial and error (this was a few years ago, so I don't remember specifics). I seem to remember it asking the backend for ANY records and then a bunch of metadata stuff, which meant having to spend time understanding not just how to answer the simple DNS TXT and A queries we were trying to do, but understanding how powerdns would interpret the client query, pass it to the backend, and interpret the result (which was quite different from straight DNS).

That also made it really hard to test, short of setting up a full end-to-end integration test including running powerdns (which I think we have, but isn't fully automated).

If I was building that service again, I'd definitely have a serious look into building it directly as a standalone DNS server rather than a backend for something else.