The AMSA magazine a few years back published that there was approximately 5% attrition at US allo medical schools and of that 1.5% was due to academic reasons. They didn't include being held back in their attrition figures, but I would suggest that its another couple of percent. The match rate for US allo seniors has historically been about 93%. Another bunch will land a spot in the scramble and many US seniors who come up short in the match will do a research year or otherwise defer graduation and try their luck in a subsequent year, so I'd be surprised if the total percentage of graduating US allo med school seniors who end up in residency isn't over 98%.
By contrast, at some caribbean schools, you might see statistics like 30% of the entering class gets weeded out along the way for academic reasons, another 10% get held up by internal exams required before they can sit for the Step exams or start rotations, and of the remainder only about 70% end up with a bona fide residency slot. So it's not a 70% success rate, it's really a 70% of 60%, or a 40% success rate, as compared to the US allo's 90+, even at some of the better offshore programs. That's better than a zero percentage chance, so if you have no other shot at being a doctor, that still might be enough to run with. But few are in such a deep hole that they can't at least somewhat salvage things. And more than a few folks who are in a rush to get to med school rather than fix things are precisely the folks who won't prevail in such paths.