IMHO, listen to criticism, evaluate your own training, let the criticism go.
Everyone's got an opinion. It is very common to defend your art by knocking others.
From what I've seen, MMA can beat most people in a sparring match. They are tough fighters. If all you want is one-on-one go-to-the-ground, its great. It has its limitations just like any other art.
Now get back to the mat.