Most of the regional training programs were originally designed for 250 hour FO's to be successful. In most cases, the training programs are the same. In theory, any 1500 hour pilot should have no issue in training.

The biggest difference between success and failure has been instrument flying. Those that have had a good amount of instrument flying tend to be very successful. Those that go to 141 schools that do not allow flying in actual IMC are not as likely to do well. Same goes with the private pilot that will never fly in actual IFR.