Hmmmm Questionable!
I’d question how the title “wing chun expert” is used. IMO, I don’t think there are many experts of anything these days, seeing how people especially MMA guy jump from style to style. MMA think should really be called modified greco style wrestling only because it has kicking and punching added to it, which is no big woop!
Also, from what I see too, any art where it’s primary objective is to break, mame, or kill someone, like wing chun, will not fare well in any submission style contest, unless you happen to be a jiu-jitsu or some other form of grappler for which these contest and it’s rules support mostly, IMO.
For example: I found for instance that most grpplers can take a punch. So if I was the wing chun guy, not trained in submission, or locks, etc, then my only option would be to go for the guy’s vitals in order to stop him. That’s how we are trained and also that’s how I would approach it in the street, but as we all now, you cannot do arm breaks, jabs to the eyes, small joint locks, or do anything to the knees! The very arcenal of weapons a wing chun guy trains with is eliminated and stripped down to have to fight the wrestlers game. You might as well tie their hands behind their back, Which is why I say, you will never see, say an expert, kung fu practitioner or real combat practitioners competing and or able to win in this kind of environment. Now if we were to take it to the street, that may level the playing field somewhat so the Kung fu guy or street combat guy has now a fighting chance, and is able to use his very unique and specific techniques that otherwise are banned from the ring. Think about it, If I was the striker going against a grappler, I will not want to go to the ground, and if the guy can take punches and is much larger than me, I’ll want to take him out the fastest way I know how, and that is, go for his vitals upon first contact. Also, if I see that he is on the ground and I’m still standing, I’m not going to jump omn top of him as I’ve seen many grapplers (UFC mostly) do to continue and beat the guy, no way, I’m going to stay up and stomp on his head, his ankels, his knees, etc…, where I still have the most advantage. 
I think the main assumption being made by many MMA enthusiast is that UFC/MMA is actually limitless and all encomassing of many arts mixed together, and it really is not. Not while there are still safety issues involved for the purpose of sport, and those rules and regulations kind of already dictate how one can only be defeated and what methods you are actually afforded, in this case, it’s grappling! Unless of course the other guy can’t take a shot to the head, but if he can, as many seem to be able, the odds are in the grapplers favor 3 to 1 perhaps more, and would explain quite a lot to why many kung fu practitioners don’t do well! Besides other more common and obvious factors like, not a real expert or compitent fighter at all which would be my first guess!LOL, but it wouldn’t matter anyways, the deck is stacked against them no matters if he’s a decorated special armed forces combat guy who has ears of his victims worn around his neck and who has seen more action than anyone! 
