In his defense, the whole damn team is weak against the run. The DEs can't contain or set the edge, the DTs don't establish the line of scrimmage, and our safeties just don't have the strength to take on bigger runners, receivers, and blockers.
If Edmunds had an average run defense around him, I still think he's a good fit for what McD wants to do with his defense. But you can't sell out against the run personnel-wise as much as the Bills seem to be doing.
I do think Edmunds fills out his body a bit more and becomes stouter against the run both physically and (more importantly) mentally over time, but right now it's the weakness in his game.
He may as well stop biting on play action and drop back every snap since he's so bad against the run right now anyway lol.