This sounds like a blocker if the user cannot create constraints on the entity fields (e.g. @NotNull).

Its not a question of not being able to apply validations. Hibernate's BV implementation supports a feature that is not defined in the spec, namely the ability to export validation constraints to the database schema. However, as a matter of timing that may not be available if the BV is not available up front.

It would be helpful if you could somehow classify the new changes that wouldn't work without the database-specific info (performance enhancements?)

Mainly it comes down to database specific defaulting of things. Mainly type choosing and automatic quoting of "keywords as identifiers".

Are you thinking that the thrown NotYetAccessibleException exception is ignored and initialization continues after skipping (early) use of the BM/VF/DS?

The thinking was that we'd use the NotYetAccessibleException to disable certain things (automatic quoting of "keywords as identifiers". for example), but that we'd continue on with the bootstrapping.