In Search 5 we had two such classes: one for JPA APIs (EntityManager), and one for native APIs (Session).
The problem is, having two different entry points make things harder when it comes to writing the documentation. See for example HSEARCH-3400.
It also, obviously, makes things harder for new users, which could use the wrong class by mistake.
Maybe we could keep the split between FullTextEntityManager/FullTextSession and the two versions of FullTextQuery, but merge the two Search classes?
Or, if we really need a JPA-only API, we could at least add all the getFullTextEntityManager methods to the native-API Search class, so that at least someone trying to use the native APIs with a JPA entity manager will not encounter any error? But I'd prefer to stick to a single entry point, if possible.