Both products are great, at Appwrite we created the majority of the product from scratch in aim to have tailored made dev-experience which was a big reason for starting the project from the first place.
Postgres supports jsonb which I've used with Supabase to act as a Firebase analog. I just pushed all data into a JSON blob rather than setting up tables, it worked well, pretty much the same as how I used Firebase.
I'm trying to be as objective as I can, but building the entire ecosystem around a single product like Postgres ( even though tried and tested ) comes with its own downsides...
[0] https://supabase.com/