One shots are not what assassins should do as a default condition.
I'm only speaking from my experience here, but as I recall from back around s2 to around s4-ish(?), assassins weren't supposed to "Oneshot/one combo" as the default setting if their kit unless you were someone like Sona. Even someone like Zed, who when he was released really looked like the champ made to one-combo people, had to poke you down to around 60% health with his shurikens in lane before he could all-in you with his ult.
I'm trying to say that this idea or entitlement that assassins are treated like their default condition is "I should be able to kill anyone under 2.5k health if I'm even or ahead", and that's simply not good to play against. Before, when dealing with some of the older "assassins" who really were just souped up burst mages (Katarina, Leblanc, Fizz, old AP riftwalk Kassadin), they could only insta-kill you by punishing you for staying somewhere with half health. You knew if you kept your health high and stayed with your team, they would peel you and you'd survive.
No one is saying assassins shouldn't be a huge damage threat when ahead, or in the lategame when everyone's builds have diverged. They should be made to punish people (Or entire teams) who are alone, wounded, and overextending.
However, now we have stuff like Rengar, Kha Zix, Camille, and "Miss all my skillshots and kill you Zed", who have enough damage in a short span that they're nigh-unpeelable. The counterplay is gone.
Now that assassins are entitled to the 100-0 one shot as long as they "play well enough" (Let's really be honest here, league is not a very mechanically challenging game, not even when playing certainlyt champs), the whole class becomes unsatisying to play against. Combine the damage from new runes and Zed becomes basically AD Annie, who as soon as they hit R will nuke you for all your health. It sucks.
Tl:DR : Assassins shouldn't one shot and the attitude that they should is what makes them suck to play against.