While I do think that some martial arts really are better than others, I think that individual skill can really help to bridge the gap. But I don't care how good you are at Tai-Chi...
The problem is that most kung-fu fighters I've run into (and I know several people who train it, including two who are sifu themselves with fairly large classes) are convinced that kung-fu is all one needs. They get hooked on the spiritualistic, ritualistic, and forms bullshit and even the ones who are pretty good at kung-fu point fighting end up being essentially terrible fighters in an MMA rules fight, especially against anyone who has a ground game.
I think this is a problem largely with the kung-fu community and mindset and not the art itself. For whatever reason (perhaps better penetration of MMA into Japan and thus Japanese martial arts) Karate, Tae Kwon Do, and Judo seem to have made much more successful transitions into MMA- each of those arts has representative fighters in the higher echelons of MMA. Not so much Kung Fu.
I'm not trying to be a smart ass here, I'm really not. But what TKD fighters do you know who are at the top level of MMA? I've heard of a few fighters who trained TKD as kids, and one fighter who said he wasted his time earning a TKD black belt, but that's the extent of my knowledge.
I know of a few top name fighters who trained Karate as teenagers GSP and Liddell. And of course Machida who still trains Karate.
I think most of us can name some Judoka.