I would say any use case that fits in with Firebase could mostly be done with Supabase (barring cloud functions, at least for the moment).
The only significant limit is the lack of RLS/Auth on sockets, which limits what you can safely do on the front-end. Hopefully they'll be able to get that fixed eventually.
Examples:
- Social media platform
- Slack clone
- Datastore for an API
My current use case is for providing public transport data, accessible to developers with an API key which they can generate. Depending on their subscription plan (handled by Stripe) dictates what data they can look up as well as how requests they can make.