They're Abominations, a bunch of different things rolled up into one. Giving them less weaknesses and counters and resulting into being over powered. Usually comes to a champion that is remarked with more than one tag. I've heard Jax as a diver, slayer, skirmisher, fighter, and even a tank and Juggernaut. Honestly I find all those descriptions accurate and one of his core problems.
Pyke was a support assassin. Now he is this support assassin, slayer, skirmisher, that counters his own counters, tanks and especially with his ult. It only starts from there, as he can kill everything.
Kai'sa is a Vayne copy, which Vayne is another Abomination being an Assassin ADC that pretty much counters everything, can 1v1 anything, and goes god mode late game. Vayne's only possible balancing factor is that she can have a weak early game pre level 6, which can be exploited. However people have a hard time closing games in solo que, so she can thrive. Kai'sa is the samething being her copy, but without a weak early game.