With all the tenacity runes available, mercury treads and elixir of iron, you can get 76% tenacity.
And you can get rooted / stunned by champions like Morgana, Nautilus and Veigar for approximately 2 entire seconds.
Are you telling me 2 seconds is 24% of the full duration of those crowd control effects?
iirc tenacity does not stack additively (barring a small number of combinations), but rather stacks multiplicatively. So no matter the amount of tenacity they introduce it would be impossible to get 100% tenacity. For instance Garen gets 60% tenacity just from his W alone, so Legend Tenacity+Mercs would put him over 100% if it was additive.
It has a reason why people say
This Morgana Q roots for ages..!
Tenacity or no basically every cc in this game lasts longer than it should.
As someone pointed out, tenacity does not stack additively. If it did it wouldn't actually be that hard to get over 100% tenacity and become completely immune to crowd control. It works multiplicitively. Here's how you calculate it:
Morgana's Q is capable of rooting someone for 3 seconds at rank 5.
You have 
Mercury's Treads. Its ability lowers this duration by 30% meaning only 70% of that 3 second root applies (which is 2.1 seconds)
On top of Merc Treads, you have a fully stacked Legend: Tenacity rune. This lowers the duration of the remainder of the root by another 30% as it is a different source from the Merc Treads meaning the root will now only last for 1.47 seconds. By contrast if tenacity were additive then the root would last for 1.2 seconds instead.
It took me a while to figure out how this works, too. Like armor, additional sources of tenacity give diminishing returns and as such there isn't really much point in stacking all the sources you can get your hands on unless you're having a MASSIVE problem with the enemy locking you down with all of their CC. If you're, say, 
Darius, then Merc Treads and 
Sterak's Gage should be all the tenacity you need (With an Elixir if those two are still not enough).
I've never noticed a difference with it, but I do notice when I don't take Bloodline or Revitalize, even in the early game.
Post clip...
You are just making things up.
Any champion can get %77 Tenacity from mercs, Legend:Tenacity, Elixer of Iron, and fully stacked Unflinching. (This will just begin to scrape the .5s Tenacity hard cap on a 2s CC.)
Without the Unflinching it's still a reliable %63. We haven't bothered to include Sterak's procs, or Cleanse.
That's %63 Tenacity from 3 sources that are pretty easy to have all together by midgame.
Smite DOES proc Unflinching. You can maintain a fairly consistent (well, on-demand, but easy to get) %32 Tenacity without mercs, just by running Unflinching and Smite.
The %10/%20 component of Unflinching does stack additively with Elixer of Iron. One advantage of Unflinching is that you have full use of it lvl 1. Legend Tenacity probably takes about 12m to fully stack.
Math? Ha!
Tenacity Runes don't start off fully stacked, and you won't have Elixirs of Iron until super late game. Having all three at once is highly unlikely.
Tbh i found buyng tenacity pointless.. With this ammount of cc we have in this game, it doesent make any difference. You will get hit by 1 cc, there will be more on the way, and even 1-2 seconds of being unable to move is death sentence.
They need to reduce Knockback, bar certain displacement abilities such as grabs, hooks or throws.
Whatever they work, they look pretty underpowered, as the current (2 year old) burst meta establishes.
The reason Tenacity is worthless is because so many CC abilities aren't affected by it at this point.
Same reason resistance stats have become less valuable over time, as there's an over abundance of true damage in the game.
math
The Morgana stun always feels like it takes ages because of the hitbox is bigger than what is telegraphed visually.
Same with Nautilus' hook.
Iirc, All CC effects cannot be reduced below 0.5 seconds even with 100% tenacity.
They do work. The problem is, generally Tenacity caps at 51% unless you are some champ with an additional tenacity based skill.
P.S. You're imagining things if you think Morg on 73% Tenacity roots for 2 seconds. More like 1.2 or so seconds.
tenacity has a limit like CDR has a limit
i dont remember the tenacity limit
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they work they just aren't useful where you think they would be useful. and they are useful where you wouldn't think they would be useful.
It does
I mean, unless they are, in fact, bugged, I think you aren't actually being stunned for 2 seconds. And if indeed they don't work, there is a "Report a Bug" section in the forums.
