Also, legendary creatures can do legendary actions; if they aren't attacks, there is no penalty on it.
With incapacitated they cannot.
A creature could have dispel magic, and just drop the effect without doing a save. A BBEG could have a mook with dispel magic, and ready an action for after it goes off.
It is already really strong, in that it can give your entire party a turn of advantage attacks and kill a boss monster's action. The fact that it is strictly weaker than incapacitate is good. And as a 6th level spell, you are probably only going to see it well after system mastery has arrived, and then only 1/day until like T4.