Nearly every assassin in the game is garbage late. I can confirm this to be true.
Just got out of an almost hour long match in which all members of my team went assassin except our ADC who picked Kaisa.
We murked the enemy team so freaking hard early on. Our Kill lead over the enemy was more than 20 at one point. Eventually, however, the enemy team scaled into the mid game and my team imploded for lack of a better word. Our top lane Rengar never died once throughout the match, but he virtually gave up oe winning and engaged in his own mini game of "how many times can I kill the enemy soraka and get away with it", spoiler alert, the Soraka ended with double digit deaths, most of which were a result of Rengar ulting her then running away.
I was playing Pyke and between the enemy Jhin reaching legendary status and the tanky Garen with a healer support on back up whenever Rengar was waiting for his ultimate to come off cooldown, I was rarely able to execute anything. Kaisa kept getting caught out, and the enemy team managed to exceed our kill count after a while with the enemy Jhin getting to legendary status.
Things were rapidly falling apart for team Assassin and I needed a plan. I noticed that we managed to take most of the enemy's turrets early and that the enemy had taken none of ours since they were preoccupied with killing us whenever they spotted us And our jungle Khazix wasn't fairing well in fights, but he managed to take four drakes plus secure dragon soul for us, baron twice, and elder once.
The enemy team was obviously not interested in objectives, taking or protecting them. Realizing this, I opted for a league of legends classic, and played hide and seek with the enemy, baiting them away from their base by making myself visible to them long enough to lure them outside of base. When I managed to get them to chase me far enough, I disappeared in their jungle and used my superior mobility to invade their base and take out an inhibitor. I repeated this process until our eventual victory.
Long story short, we should have lost and won only because of low elo players' mindset that kills matter.