Why is it many martial artists can't stand when it comes to UFC?

It's because their own martial arts are now exposed as incomplete and not effective. Look at the other guys answer , eye gouging , ear pulling how bout we add some wet willies and nookies in there too? You keep your b*tch fighting tactics while you get smashed in the face with a Muay Thai knee or get taken down and dumped on the back of your head. You know how hard it would be to actually use that eye poke technique in a full contact fast pace fight? And small joint techniques suck because they do not cut off the rest of the body like a good armbar , or triangle choke does. You try to twist my pinky, I will smash u in the face with two strong punches , and see who delivers more damage.
As a former student of crappy martial arts , I love the fact that the UFC is big now. All those bullshit gyms who do no conditioning , no full contact sparring , no weight training, no grappling can not scam people anymore, because they have seen what works and what does not.

Many martial artists were trained to kill or inflict serious injury. Are they supposed to do so in cage fighting? Can you really bring yourself to gouge out the eye of a BJJ fighter, just because he got too near to you for a takedown?

Ring fighting favours the wrestler and the grappler, because these are not death encounters and there are clearly rules in place to bar eye gouging, groin-grabs and yanking of ears.

Please remember that in ring fighting, small-joint locks are banned. And most martial artists are trained in small-joint locks.