> A user of these IDs my not realize that the reliability of the ordering can be substantially reduced depending on where the IDs are generated.

That can only be addressed with improved documentation and shared understanding of the subtleties and pitfalls of distributed time synchronisation.

> Some applications may be able to tolerate inconsistencies in ordering, others may not.

Indeed. Proper thought must be but into this sort of thing. ULIDs aren't an exception nor a silver bullet.

> How many bits of approximate time does one really need?

Entirely application dependent.