The number of instant-cast, AOE knockups is far too great for there to be no itemization available to mitigate their effects. Period. We are playing a much different game than we were when Tenacity was designed, the amount of CC that is unaffected by Tenacity is staggering, and it's time to at the very least TEST adding knockups to the list of CC effects reduced by Tenacity on PBE and see how it pans out.
KFCeytron5 votesKnockups exist because they aren't affected by Tenacity, not in spite of that. Most CC has to be balanced around use against champions with as well as without Tenacity, similar to how heals might be used on champions with or without Grievous Wounds. Knockups don't last as long as other CC due to this consistency.
DOUBLE TAPPED E1 votesThen there need to be fewer knockups. They interrupt channels, and are essentially a stun, which allows other champions to easily hit their (more powerful because they are affected by tenacity) CC, and chain them together until a target is dead, without them being able to do anything at all, even flash or Zhonya's. At the very least, airborne champions should be able to flash or use other mobility spells.
For reference, these champions have this number of knockups / knockbacks (airborne effects):
1 -
(wall displacement) 
(Q counts as a "suspension" so she only has 1 knockup, technically) 
(pillar displacement) 
(with upgrade) 
2 -
3-
Idk, that seems like a lot of abilities where the only counter play is mobility. You can say flash, but more often than not there are at least 2 of these champions to deal with, if not more.
Mordepool1 votesIf the duration of a displacement is long enough to blow a champion up ( which it is) then there is no difference between that and conventional cc
JackMcSnipeyz2 votesThat's because displacements are more dynamic than stuns/roots. How do you use tenacity on maoki's knockback? It's duration is tiny already and how does it effect the displacment's range? Do you suggest a tenacity stacked champion (like garen) will just barely move? That will completley break maoki (for the worse) and he will require compensation buffs.
Draven's E, Taliyah's knock back, Syndra's scatter, vel'koz E - and many other similar spells were balanced without tenacity in mind, they are balanced around themselves no matter the opposition, so if any knockup is strong, it should be looked at individually.
DOUBLE TAPPED E1 votesAgain, the issue is not the power level of any individual displacement effect, but the number of them overall.
Mordepool1 votesYup considering you get displaced and stunned. There is a reason why Champs with Displacements are always Meta.
There is no counter build. So only champs with mobility get a Counterplay.
Which is bullshit.
Tahminatrix 1 votes(and knockbacks)
Yea this is another one of those balancing messes, sometimes I would run towards the nearest wall and HOPE vayne e`s me into it since the stun is so short compared to the distance I would have to walk (if im stacking tenacity that is)
Also lets not to forget the champions whos entire power budget is put into having knockups, whenever someone has a knockup, that tends to be there entire identity as a champion.
Malphite wouldn't be malphite without his knockup, same with rakan, same with alistar.
Its literally as if they are a part of an entire class of champions, the knocker-up-ers.
If they found a way to make some knockups less important, these champions would be allowed to be more than knock ups.
ContDeCaterinca11 votesWhat's the average duration of a displacement compared on a stun?
Ashe even has a 3.5s stun, the longest knock-up has to be like 1.5s.
Displacements are balanced around having no counter, but having short duration, suppressions are balanced around the user not being able to move or act in any way while their target is suppressed.
Mordepool1 votesThats not very short considering the current time to kill is 2 seconds.
Tormentula1 votesI think like surpression, it has more to do with the users design rather than the knock up concept.
For instance, tenacity doesn't affect malzahar or skarner because the CC is based on their spells themselves rather then how much you cancel them.
Two examples that come to mind for knockups/displacements are thresh and blitz, which wouldn't pull properly if the duration wasn't exact. Even sylas has a whole animation built around landing his and pulling himself in.
I feel they need to add more subcategories to knockups and displacements, ones that can be reduced by tenacity (cho Q, Galio ult, malphite ult, etc, stuff that seems obvious) and ones that shouldnt be for gameplay purposes (or balance).