Please note that being "broken" =/= "OP"
But some champs like Azir, Kalista, Yasuo cannot be balanced through number changes and it sucks because they're either dominating their lane or UP due to balance changes and it sucks!
Why can't we get another Vel'Koz? Most recently released stable champ.
Nameless Voice1 votesNone of the champions that you've named were even released in 2015.
Are you saying that Rek'sai, Bard, Tahm Kench and Ekko are inherently broken and cannot be balanced?
Clint Orris1 votesLook at Rek's balances since release and tell me if that's ok
Look at Ekko's kit
Bard is alright
And it's too early to say anything about Tahm.
Nameless Voice1 votesRek'sai is numerically too strong because she is still too useful while building full tank. Numerical balance.
Ekko's kit has tons of counterplay built into it, regardless of his current power level.
They are in no way in the same league as Kalista and Yasuo in terms of poor design.
Telemachus71 votes
is stable champ
the new 
is nicely balanced and stable
i think in the cases of ekko, tahm, bard, and rek is that they tried to do something unique with these champs. Which they did. That might cause some balancing challenges.
The new 
was broken as hell when it came out, but they fixed him and he is in a pretty good spot i think. 
was a great change too, she fits into the game much better and has a nice identity.
was buggy as F when he came out. but they fixed him after a few patches.
Deathstr0ke131 votesMost champs are stable. Every brand new champion is bound to be a little too strong at first, but that just takes time to balance out, like Ekko's ult being mana-less. For the most part though a champion that is "OP" has less to do with the champion itself than it has to do with the player simply playing the champ well, or more so just the other players just simply not knowing how to counter that "OP" champ due to lack of experience/knowledge of the champ
Knowledge is power