Transactional tests have some downsides, unfortunately. If your tests test transactional code, that code itself cannot create transactions; they have to use savepoints, which aren't quite the same. Transactional tests also don't work with testing anything concurrent, unless you share the session across threads/goroutines/whatever.

Lastly, if a test fails you'd typically like to leave the data behind so that you can inspect it. A transactional test that rolls back on failure won't allow that.