JPA only support int / long for IDENTITY / AUTO/ SEQUENCE AFAIR

Do you have a reference for that? I had a (short) look into the spec and couldn't find anything. If that's the case, then that'd be a pity indeed.

It makes the migration of an application from an RDBMS to a NoSQL harder

Harder in which way? It's my understanding that when using IDENTITY and moving from one RDMBS to another which doesn't support that strategy, I'd get an error as well. So I'd either have to change it into a supported strategy or use AUTO. I think it'd be the same when moving to a NoSQL store not supporting the currently configured strategy.