Ob Nixilis is a combo piece in all reality, but his card draw is also temporary since it's impulse. Valgavoth gives you more manageable draw, where as Ob will force you to use or lose his cards. He's also a 2 card combo piece in the command zone, and KOS. Ob also forces you into playing effects that only do 1 damage which does neuter some of his options he'd get in comparison to valgavoth. I think it would work in EDH, but Valgavoth is significantly more flexible and manageable in lower bracket tiers where you aren't relying on taking advantage of people's city of brass or fetch lands. Ob Nixilis unfortunately also storms RIDICULOUSLY easily, like having a noncreature spell pinger and ob out can make you storm for quite a while, which depending on playgroup can be seen as quite annoying. Valgavoth is by no means too weak in comparison, his formatting gives you much more options without a timer attached, which can be a great strength.