The first two problems are nothing, IMO. Server <> client sync is handled by your engine or, if you need to roll your own, it's not that hard. Syncing to a database is more tedious than difficult.
Getting rid of a game engine on the server side adds a lot more work. I don't think it would even balance out.
Having implemented the first two for a web game i absolutely guarantee you those are far from "nothing". It takes considerable diagraming & handling to coordinate FE + API on state changes.
If they in fact can solve that (haven't checked myself), it would be a massive time saving for development.
Maybe I'm just used to it having implemented it several times. But my point is that they require a relatively fixed amount of effort. Once it's implemented you shouldn't need to touch it again.
1. Server <> client sync is handled for you 2. Server <> database sync is eliminated 3. Deployment is super easy because you just upload your wasm module to the database, and SpacetimeDB schedules it (similar to Kubernetes honestly, but at a different level of the stack)