There is a big difference between in architecture between Neon and PlanetScale, CockroachDb, and Yugabyte. Neon is shared storage (storage is distributed but shared) and the others are shared nothing. Shared nothing systems are hard to build with supporting all the features of the base system. E.g. https://vitess.io/docs/13.0/reference/compatibility/mysql-co....

Neon is 100% compatible from Postgres b/c we didn't (or almost didn't) change the Postgres engine.