It was bad enough when merc treads and Zephyr were the only Tenacity items...
But now the only tenacity available for purchase is 20% from Merc treads.
Speaking generally, I don't know how else to say it other than it is just not enough vs all of the cc's in the game, especially those with 2-3 second hard cc's, and champs that have 2 hard cc's to chain.
E.g. In most cases a player who builds tenacity should expect the ablility to escape a single enemy when caught by their cc, but likely not from 2 or more enemies.
I feel as though Merc treads don't even provide the necessary cc reduction to escape most catches from just a single champion anymore.
I also feel like I'm the only one who see's it like it is, a flat 15% buff to basically all crowd control abilities and champions with anti-cc kits like Olaf, Irelia, and even things like wind wall or braum shield are indirectly buffed.
Morgana super buffed with her huge cc and anti-cc kit.
Tenacity should not be nerfed for everyone just so it can be an "option" for full tank mastery's.
and
you'll only get the 35% from Moonflair Spellblade.
Same with every item with a UNIQUE Passive. Any effects (like increases in AP or Movement Speed) that aren't classified as UNIQUE will stack, but any that are classified as such will not.