You can't CC most elite mobs, some bosses are even immune to taunt.
Some of the currently taunt-immune bosses (Mandalorian Raiders, for instance) are actually bugged at the moment and are meant to be Tauntable.
Most elites are CC-able. The first ones I ran into that weren't were on Taral V, but the next FP after that (Maelstrom Prison) were CC-able.