When a player gets immobilized you cannot do any actions, can't drink potions, cast abilities etc until you break free (at least for me). That in essence makes it better than stuns and able to applied far more often.

The DKs fossilize, and dragon leap abilities are very glitchy and often causes the player to be unable to break free unless they spam the buttons in which case another bug happens, it drains all your stamina as if you had actually broken free. I had an entire bar of stamina drained from this glitch.

I have had this happen to me non-stop in BGs the past few weeks. I thought maybe I was imagining things, but it is easily repeatable.