Both generally and historically the success rates are very high. Right attitude plus right training works.
Line training however is effectively doubled these days for first time jet and heavy turbo prop entrants. Again, right attitude and trainers pays real dividends here in building up awareness and judgement.
But this is more costly in terms of sheduling a finite number of trainers especially in a company with a high proportion of long sectors. It takes a lot of sectors and a lot of time to get you signed off on line.
I'm not aware of anyone ever stepping up to the plate and explaining this to wannabees. With new aircraft and training credits it is effectively far cheaper in terms of scarce company resources to type rate an experienced pilot who's flown a twenty tonne plus aircraft than to welcome you on board with your freshly purchased type rating in your moist little hand.
Search is on for the weekend and you're welcome to have a look at my posts. Lots of opinions and war stories around but you'll find I have never recommended buying a type rating - I've just explained the reason. If this isn't absolutely clear to you I'll be happy to explain point by point. There are no dumb questions in this business because we're talking about your money not mine
Regards from the Towers
Rob