Managed database offerings give you a Postgres URL and you just go to town. They take care of scaling and monitoring. Fly Postgres is automated, but not managed: our tooling will boot up a Postgres cluster for you, at a specified size, but it's not going to do so much database-level monitoring that you can forget about the database and just assume it's always healthy regardless of your usage.
If your expectations are set from Heroku's Postgres, you want a managed database. You're going to notice that Fly.io people aren't going to push you to Fly Postgres. You want to be reasonably comfortable with clustered Postgres to choose our unmanaged Postgres over Supabase's --- maybe not so comfortable that you'd set it up yourself (it's automated, after all, and does a bunch of cluster orchestration for you) --- but enough to do your own monitoring, sizing, and provisioning.
Lots and lots and lots of people rely on Fly Postgres, and it's a reasonable option. If I was doing a low-level project, like an individual service in a larger ensemble, or an expiriment or side project or a spike, I'd probably use Fly Postgres. But if I was launching a whole product on top of Fly.io, and a Postgres database was my system of record, I'd want Supabase.
If your expectations are set from Heroku's Postgres, you want a managed database. You're going to notice that Fly.io people aren't going to push you to Fly Postgres. You want to be reasonably comfortable with clustered Postgres to choose our unmanaged Postgres over Supabase's --- maybe not so comfortable that you'd set it up yourself (it's automated, after all, and does a bunch of cluster orchestration for you) --- but enough to do your own monitoring, sizing, and provisioning.
Lots and lots and lots of people rely on Fly Postgres, and it's a reasonable option. If I was doing a low-level project, like an individual service in a larger ensemble, or an expiriment or side project or a spike, I'd probably use Fly Postgres. But if I was launching a whole product on top of Fly.io, and a Postgres database was my system of record, I'd want Supabase.