They basically said that the training is a lot less structured and there's a lot less support available for trainees. Because US teams are very lean, trainees end up doing a lot of the grunt and grudge work because there's nobody else to do it. Whereas other firms have more staff so it means trainees aren't lumbered with (too much of) the boring work. The culture at US firms also tends to be a lot more cutthroat and they have less tolerance for people who take longer to develop. If you know what you want to do and are prepared to work extremely hard, the US firms are it. If you're unsure, it might be a baptism of fire which you don't recover from. They didn't explicitly say "don't apply for a US firm", but it was more of "know what you're getting into." US firms are also constantly trying to poach their talent, which is a further annoyance. Then again, US firms have massively disrupted the City law market and have become serious competitors to MC firms, who used to have a monopoly on being the pinnacle of the big law legal profession. So there are likely elements of bias, but their perspective was interesting.
I met with another lawyer last week over lunch who trained at
Freshfields. They now work for a regional firm, but they advised training at the best law firm possible because it opens doors down the road. It's much easier to move from MC/SC/US to a national or regional firm than the other way round.