Muay thai sucks in mma. Conor killed the Muay thai train.
Anderson Silva, TJ Dillashaw, Jose Aldo, Renan Barao, Carlos Condit, Donald Cerrone, Shogun Rua, Wanderlei Silva, Joanna Jedrzejcyk... They all suck.
Conor McGregor must suck too since he was working with a Muay Thai guy yesterday.
Muay Thai is actually very effective, and if you put a high level strictly Muay Thai guy vs a high level strictly Karate guy in an MMA fight, the Muay Thai fighter will most likely win because he will have too many tools. Leg kicks, elbows, knees, uppercuts, hooks, clinch fighting. The Karate fighter will be overwhelmed every time.
Unless you train the Karate guy how to deal with and defend against all of those (which would be teaching him Muay Thai), then the Karate guy will pose a threat to the Muay Thai fighter.
Karate and TKD have a lot of useful things to offer, but if you could only learn 1 stand up art to compete in MMA, you are much better off with just Muay Thai than just Karate. What's great about MMA though, is that you don't have to limit yourself like that.
Look at TJ Dillashaw for example, he is definitely a Muay Thai fighter, but he has taken Muay Thai footwork and movement to a completely different level. Look at Anderson Silva, he is a Muay Thai fighter but he also implements more elusive footwork and crazy head movement which is not found in traditional Muay Thai. Jose Aldo also implements boxing head movement. Conor McGregor uses a karate stance with some karate and TKD kicks but he also punches like a European boxer, etc.