Hi… The biggest one is that the Fly.io Machines platform is hard for databases, in general. Even world-class experts like Turso and Supabase throw in the towel, 
, sometimes.
For experience with MariaDB/MySQL specifically, I’d suggest browsing the mysql tag, here in the forum, where there’s a person who nearly lost an entire database, etc.
(Actually, that happened to the same person twice, as I recall.)
The official doc about the underlying architectural problem is here:
https://fly.io/docs/apps/app-availability/
Individual Fly.io employees have put it more strongly:
(That thread was about Postgres, but ultimately those principles apply equally well to MySQL.)
Consequently, unless you were specifically interested in running a multi-Machine database cluster, the non-Fly options are really the way to go.
(The new Sprites concept may change that balance eventually, but it’s still under heavy development.)
Aside: If you are interested in clusters then Fly.io is really nice, though. One of the main things that I use it for is to experiment with multi-node databases…