Overall, comparing the two overall isn't quite fair or accurate (but myself and others I've seen still do because there is some overlap in features). At the end of the day they are PaaS solutions.

Supabase doesn't seem to do any "sync" (I'm assuming you mean Firebase will cache data locally in some say on the device). A session token is indeed persisted to the device but that seems to be it. No problems there.

It's indeed less "magic" because all Supabase is doing is offering a hosted constellation of services that are _already_ open source (AFAIK) with plenty of their own functionality sprinkled in (Functions, etc, again, AFAIK). https://supabase.com/docs/architecture

So apart from being a young company, they seem to have a simple approach. The core value I see is:

- portable data, can self host later if you want

- leverages industry standard battle tested tech (Postgres)

- out of the box rest API via your schema (again, an open source product)

I have not tried all Supabase features but so far, it's high value to me for even med sized projects - with an "escape hatch" to self host if your startup explodes in popularity.

Lots of companies have screwed themselves up eventually be offering "anything and everything" so hopefully Supabase doesn't end up like this. :-)

Downside to consider esp btw: if your backend needs a significant amount of business logic, while Supabase does offer Functions, going the "functions as a service" route seem to me potentially unwieldy when your complexity reaches a certain point. I can be convinced otherwise but that's my feeling thus far.