Actually talon is balanced too (just one or two slows, no escapes, has to commit 100 to a kill or he dies), Kha is incredibly balanced (Escape which is also his engage tool, 2 weak slows, only strong when enemy is alone, if he doesn't kill you he's got no escapes), Katarina is too (low range, escape which is also her engaging tool, can only burst you down and survive or try to and die miserably, no cc).
Also talon need his ult in his burst and unless he's ahead he won't have enough to kill you AND escape with it.
All the assassin's I've mentioned have very clear counterplay:
-Talon: almost no range or poke, engages are pretty telegrapehd (he will ALWASY appear behind you so you can easily cc him out of the engage) also his cc only lasts 1 sec even if it's 99% (which becomes 70% since there is an hard-cap on ms so every slow stronger than 70% isn't actually stronger than a 70% one), needs kills to scale and falls off later in the game against tanky teams;
-Katarina: almost any champion in the game can stop her ult and engage, she has fairly low range, dependant on bursting you down, no cc, no real poke (unless ahead), silences, stuns and suppresses are a death sentence to her, no safety during lane phase unless pretty ahead. Falls off against tanky teams, can't engage by herself or she will just die;
-Kha'Zix: weak sustain in the jungle so he's weak to invades, weak ganks unless the lane has hard cc to back you up so ward counter him hard, low damage unless enemy is alone so when ganking a lane he won't have full damage, no escape unless he kills you so he's weak to counter ganks or champs with strong 1 vs 2 turning potential, extremely weak to every form of cc, only got 2 weak slows, falls off against tanky teams and needs someone to engage for him or he will just die;
Also ekko can apply his burst in 1 aa-q-aa which is incredibly fast to do, and then throw his stun to make sure the slowed enemy can't escape. Also he can use his ult for both if he's smart and escapes a bit and can use the ult to bait into his w. Also strong duels even though he's a mage.