Back when XE had branded ops, I flew several times with pilots who were young. I don't think that this is a problem in and of itself. I did, however, experience some issues with how cabin pressure was handled. Instead of releasing it slowly, one crew effectively went from cruise to landing in about 30 seconds. For someone with sinus issues, it was incredibly painful.

Aside from that, I've never had a problem with regional airlines and crews. I still prefer mainline aircraft for comfort reasons, but I don't think I will specifically avoid regional aircraft for fear of poor crew training/experience.