RBS is "insecure by default" meaning if you don't set it up right, all users can access all data. And, because it's SQL, it's really easy to write a valid and successful configuration that doesn't work. You have to test it. And for views, you have to do some real heroics to get them to respect RBS.
PostGREST is very very basic and far less powerful than just writing SQL. I found it pretty frustrating to use, knowing that SQL would be far simpler.
Yes, I know that I could make a direct SQL connection to the database, but at that point, it would've been easier to put a Rails app on Heroku and use their offering, which is far more mature.
The Supabase dev environment was pretty slick and worked well, though I had to docker implode a couple times.
I wish them luck, but I could not figure out what the use case was for their product, given that Heroku has an awesome managed Postgres and RDS is there if you have more knowledge and want more controls. I don't see how you could make a complex app that is front-end only with Supabase as your entire back-end.