300-hour wonder pilots
Ab initio programs work, but it helps to have good material. I instructed at MAPD ten years ago and, before that, at FSI training Alitalia pilots. The Alitalia guys were hand-picked individuals and mostly flew really well and kept up with the course. They had to keep up because they received no extra training periods. We had one who was washed out.
I am 100% positive that MAPD students are accepted primarily on their ability to pay and not because of their potential. I had at least three students whom, I'm sure, went on the Mesa and became captains. Two were extremely sharp; the third was alright. I had a couple of others who had bad attitudes. I also recall one who was not my student who was pushed along and was barely safe. I don't think she made it.
MAPD students are imbued with Mesa procedures from their first day of training, but once they get to class they're on their own. I know that Mesa is not shy at all about washing out people from class. Moreover, MAPD grads have to take the same 121 ride as more experienced street hires. Both of these points should be dispositive of the competency question. MAPDers are on probation the same as everyone else, and very well may be subject to the associated politics of probation as street hires.
You can produce a technically-proficient pilot in 300 hours in a quality program, but expererience is the best teacher. Everyone was new at one time, including Rod the Captain. Maybe he forgot that. Good captains teach. They show newcomers the ropes. Suggested reading would be the first few chapters of Fate Is The Hunter, and how Ernest Gann learned from his early captains.