Esker
Hero
Any BBEG worth the label is going to have legendary actions as well. One common legendary action is to move without provoking opportunity attacks (and since that's not the disengage action, Sentinel definitely doesn't help there).
Also, the reaction attack has to hit to reduce their speed to zero. A BBEG is probably relatively hard to hit, especially if they're facing a group of PCs solo. But failing that, dodge. In this case (or if you buy my interpretation of Sentinel's disengage text, which, having gone to read it, is at best ambiguous) the combo has managed to deny the BBEG an attack action. But that seems fair to me for the price of two feats.
I guess what I'm saying is that if you have a BBEG that has to be within 5' to hit, doesn't have the ability to avoid opportunity attacks outside the disengage action, is relatively easy to hit, and is facing a group of PCs solo, then if the PCs defeat them easily, it's not because of a broken feat combo.
Also, the reaction attack has to hit to reduce their speed to zero. A BBEG is probably relatively hard to hit, especially if they're facing a group of PCs solo. But failing that, dodge. In this case (or if you buy my interpretation of Sentinel's disengage text, which, having gone to read it, is at best ambiguous) the combo has managed to deny the BBEG an attack action. But that seems fair to me for the price of two feats.
I guess what I'm saying is that if you have a BBEG that has to be within 5' to hit, doesn't have the ability to avoid opportunity attacks outside the disengage action, is relatively easy to hit, and is facing a group of PCs solo, then if the PCs defeat them easily, it's not because of a broken feat combo.