Ney tends to get rated as a "excellent subordinate, poor commander" sort - he ended up with responsibilities above his abilities, which rarely turns out well. Not that he'd have been the first capable officer in history who ended up messing up due to that...

I've also read that old "Bonie" himself just wasn't what he'd been anymore either - too much stress, hard campaigns and less-than-ideal lifestyle left him physically exhausted and with assorted health issues (he'd had those for a while, I understand), and started blunting his mental edge too if only out of sheer fatigue. Someone described it as "his opponents may have improved considerably by Waterloo, but he had also gotten worse" or something along those lines. Given his rather centralized way of running the army that would certainly have been a major drawback.