Docs, oh god yes. They're the anti-Stripe in this regard.
And hard-to-debug deploy-time (and sometimes runtime / uptime) issues are a major sticking point. It is hard to know what or who's at fault without asking for it in the forums, since you can't stackoverflow much.
Their support (granted it's free) is a bit of a hit and a miss; while it isn't clear what really is on their roadmap (or important) and what isn't (even though, they are more than transparent than other providers I've interacted with).
These are but pains that come with adopting a nascent ecosystem, I suppose. I still persist with Fly because it is still simpler to build certain apps on it than on any other BigCloud (bar Cloudflare).