This will be fairly simple and short.
As it stands now, fight dungeon bosses and Champions are basically a “Bring as much DPS as possible, with a little support and move around”. There is no ability to “Control” a Boss/Champion which really limits the ability to really strategize and makes several skills useless in many situations.
I don’t think there’s a problem with limiting the amount of control you can use at a time on these types of enemies, let’s face it keeping a boss/champ on constant knockdown/stun/etc would be just as boring as running all DPS, but instead of making them invincible, limit control to a certain amount per minute or give the bosses some control breaking skills (i.e. Stun Breakers) that they can use just like the players.
TL;DR – Allow Control builds to be useful in Dungeons and against Champs!