You will never see a high level MMA fight with guys spamming roundhouses, like you see with many southpaw thai fighters, petch, sittichai, yodsanklai etc. The takedowns are one issue but fatigue also is a major factor. Movement is more important in MMA than it is Muay thai, so fatiguing yourself with heavy roundhouses to an opponents guard and body is not a smart choice when it will severely limit your ability to sprawl, change levels and use footwork in general later in the fight. It's one thing to do that in a match that is only 9-15 minutes. It's entirely different when the match is 25-36 minutes