i mean i think it's ignorant to consider one art better than another, i do seven star praying mantis kung fu and i sparred and destroyed my friend who is taller than me more reach and did muay thai.
But i still not only do kung fu but know some bjj for groundfighting and basic muay thai.
and i heard that in one of the first ufc fights it was a kung fu guy in it who had a broken arm and he beat the shit out of the guy.
i'm guessing it depends how you train, if you fought a kung fu artist who trains for the street, muay thai could probably win and with muay thai rules of course
but a kung fu artist who trains for mma obviously doesn't only do kung fu