All of his abilities seem to vary too widly depending on his metrics of Shadow Energy. Some of them can't even be used at all depending not on intuition or cooldowns, but a number that varies between 0 - 100! This puts a lot of burden on knowledge on not only the player, but everyone else playing with him, and what depth the mechanic brings doesn't seem to justify the complexity that comes with it.
Furthermore, many of his skills, to make informed decisions on how to play against him, you need to be able to eyeball his total shadow energy within 5 ~ 1%, or you need to constantly mouse over him and track his shadow energy manually. This also needlessly complicates playing against or even with him, as no one's really sure when he can use what abilites or when. This drives people essentially ignore whatever tricks he can pull out and just brute-force them. This is bad, since if the people playing against Erubus can't be bothered to counter-play against him, you might as well play against a block.
Just imagine someone trying to play against him.
"Is he at 55% Shadow Energy, or 56%? He's at 62% Shadow energy now, but does that mean he'll decay right then, or will he still be able to use his ability? Wait, is he at 70% energy or 72%. Aw, sh8t, he killed me. Eff this, I'ma go play TF2"
I just don't think he'll translate well into this game. In fact, I feel he'd be much more suited to DotA:Allstars than League of Legends.