they let some CA1 and most CA2's do hearts, albeit under some supervision. Yes there are some interesting opportunities with TEE, etc...but most cards position don't requre that.
Don't get me wrong, there is a need for cardiac anesthesiology experts, but you should know that an "average" heart case is easily performed by almost any anesthesiologist/crna. Extra training is not necessary 90% of the time.
But pain and peds standout as complex sub-specialities which not everyone can just jump into.