So when knock-ups were introduced into the game it made sense that they were unaffected by tenacity, I mean they were unique and they were few. Now we have almost more than a third of the cast with some form of knock up (or displacement/airborne affect) that are still unaffected by tenacity. And the kicker: they stack unlike other forms of CC.
Looking at other problematic factors of knock-ups, they are buggy as all hell. We've seen the highlights on youtube, or the LCS plays where a champion will suddenly disappear into the sky because they were doing a movement ability when smacked in the face by some knock-up (lee sin Q sending him into orbit is a good example) which just goes further into how unbalanced they are in the eyes of other CC where if you stack them it's less significant because of tenacity and the way stuns/snares/etc work.
Would be really great if we could see some change to knock-ups/airborne/displacements in the future (or very soon) because it's just getting out of hand how little counter play there is to them (besides don't get hit).