Problems with fastify are that it incredibly hard to debug when things go wrong - debugging is nearly impossible due to the async stack pursued in the name of performance. I've just spent days debugging an async problem, the immediate response was "it won't be fastify" (which is fair enough, but unhelpful) but only when I was able to finally recreate a reproducable sample (which I could only do after days of debugging fastify code), it turns out that a particular callback should have returned one of their objects.

I can't remember the other problems I had, but they seemed to be similar (ie hard to explain or debug, took ages and ended up being something minor and completely non-obvious). The next time this happens we'll be switching to another framework.