To my knowledge only a couple of Wing Chun practitioners have ever competed in MMA. Or at least competed on the level where there is any level of notoriety attached to their performance/character. Most principly Sean Obasi.
With regard to Sean Obasi, his Wing Chun is crisp. There are Youtube videos of him sparring pure Wing Chun and it looks damn good technically. However! if you watch his MMA fights what he is doing is barely considered anything close to Wing Chun. His guard is very Wing Chun-ey, however he appears to forget all technique once he lands a punch. Also: homeboy can't throw a roundhouse kick. That's embarassing on a basic Martial Arts level.
With Wing Chun in general, it has its areas of positive effect. Its a Martial Art that, with regard to hands, entails a centered guard and very fast hand movement and straight punches called chain punches. Regarding the legs, in most cases kicks are deisgned to control range and stifle the opponent's attack, i.e. push kicks, front snaps, etc. With stances, its about rooting and moving explovely.
These are all solid principles for any MA practitioner or MMA fighter. However, their application doesn't translate very well outside of Wing Chun competition. The conditioning of the style for said comp, leaves the user very open to a lot of basic techniques from other more mainstream styles, i.e. knees, roundhouse kicks, boxing style rotational punching (as stated earlier I think).
Moral: Wing Chun has its benefits for any fighter. However it is probably not the best option for achieving excellance in most of these areas. That being said, I think a Wing Chun practitioner has as much a chance of being great in MMA as any other practitioner of a TMA. Its all about crosstraining IMO.
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I went out the way that I fight. I went out on my shield. That’s it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit