"expert tuning" can indeed be complex to verify, but I wasn't talking about the complex cases: what I mean is that there are many cases in which it's a trivial choice.

I've been making such decisions myself ten years ago, being quite junior and new to Hibernate myself, and making the difference for a multi-million project between "dramatic failure" vs "working great".

All I'm saying is that the option to decouple the "isolation level" choice on some selected entities being cached needs to be independent from the connection pool configuration and this is an extremely important feature.

If you're suggesting we need to use defaults to match the connection pool setting I'm not persuaded as it's usually harder to understand a system whose default depends on other stuff but that's ok, not a debate I'm particularly interested in.
But taking away the option to explicitly set isolation levels independently between connection pool and specific caches that's something I'd oppose with passion