To be fair, LVG has introduced some of the best players of recent times so he must have something about him when it comes to developing young players. But, as you say, it's very hit and miss. The anecdote about him preferring Thomas Kraft over Manuel Neuer suggests that, for every bit of genius he provides, he does something bafflingly bonkers by way of counterweight.
He's worked at Barcelona and Bayern who have the best youth set up around. Giving debuts to lots of youngster he was bound to get some right. There's a lot more to players development than giving him a debut and he can hardly have a case for most of the players he's been given credit for.
He's given so many debuts at United that it's inevitable his fanatics will brag about this if any of these players go on to become good. However his policy is extremely strange, he gives you debut and then ditches you a few games afterwards for a new toy. He also absolutely despises possibly the biggest talent of our youth setup (Pereira) because it makes him cool (unusual decision, like bringing Powell on against Wolfsburg - he likes those, it makes him look like a genius - or bringing a right back to chase after a goal).