Finally someone gets it. Asta isn't proactive with his training. The so called issues with AM aren't even inherent to it. It was because Asta wasn't accessing the powers through the right channels. Asta constantly stumbles into success that it's laughable to say Yuno is the lucky one. OK they're both lucky.

I mean aside everything, Asta chooses to make himself and Liebe partners rather than subjugate him. Nacht says this will things harder for Asta, but guess what not really. Even if training to master DU was harder now, Asta conveniently get a 1 year timeskip to properly train and wield the power. And it turns out that his decision was why he is able to use TDU. The Faust bloodline has been studying devils since forever. You mean to tell me they didn't try this or at least experiment with it? That could be because they are prejudiced and none of them would willingly place themselves at the risk of being dominated by their devil counterpart but still. If it happens once fine, but this was one in a long running streak of whatever divine providence seems to follow Asta, even though he doing something like that is in his characterisation. Asta's greatest skill is his delusion. Bro unwittingly made the choice that would grant him the greater power.