I agree/disagree with both of ya. If a guy sees a sidekick coming and fires in before the kick has time to gain momentum (pressing out), basically crowding the kick and smothering it before it is fired off, that will lead to a pretty easy takedown....... The sidekick doesn't allow you the advantage of getting your arms out in front of you mid kick like a roundhouse does. Which gives you a chance at grabbing underhooks, or cattle catching your opponent and sprawling for the TD defense. The narrow profile and upper body positioning on a sidekick makes it very susceptible to getting smothered and a quick take-down. My personal opinion is that sidekicks are best suited to MMA as leg kicks, stomping the heel across the front of the thigh.
That said, i do agree that a roundhouse is easier to catch, but if you were going for a takedown attempt off a side kick you probably wouldn't catch it, you would just smother and drive through it, flinging the guy on his arse! And if you throw a sidekick that is telegraphed enough to get side stepped or parried out of the way, it is an easy way to get taken down with your opponent on your back.