first of all i won III after two seasons :) i'm in II now, and i'll try not to relegate even though i'm underdog. i thought i'm underdog when i promoted to III too, but finished 4th first season. believe me, i didn't thought that was possible with salary's like mine.
i always had bigs with reasonable SB (well ofc it wasn't high strong-prominent) and i did feel that this skill is working. never had a feeling like, oh my why didn't he do anything, he has SB! never.
ofc it's a matter how trainee is generated, sometimes it's just not worth it to train some skill from atrociuos. but what i wanted to say with this small sample. you can train good players. even for u21.