And btw... the reason I like this so much better is that we do still have access to use both Ehcache (though only 3, not 2) and Infinispan through hibernate-jcache. IMO that is a much better solution.

But if a cache provider wants to customize or optimize in some way, they can do that quite simply just by using the classes from the org.hibernate.cache.spi.support package. And given Hibernate's use of JDK ServiceLoader to discover providers... all users would have to do is to have ehcache on the classpath with a RegionFactoryProvider service defined (which is all standard Java SE stuff).