I think the primary issue with champions that are newer is simply how many facets there kits contain and how many ways they can be utilized.
For example, I personally despise Jhin's kit, despite the fact that I find his personality to be sassy af. His kit begins with complex functions involving attack speed/critical damage/movement speed that not even I can fully explain to you, he has a Q that works like Sivir's ricochet but rewards him for doing something as basic as killing a minion with it, his W is a long-range snipe that, despite being moderately challenging to land, has CC built into it (which I personally think that having anything beyond a slow on an ADC is problematic in and of itself, but that might just be me), his E synergizes incredibly well with his W and itself has a slow on it, and his ultimate is identical to Cait's, simply more interactive.
I'm not saying that Jhin is even overpowered, I'm saying that he's overloaded. Sure, I can win against a Jhin as Karma, Nami, Lulu, etc, especially if it's ranked and I put forth a concerted effort to win laning and teamfighting phases. The point I'm trying to make is that he has capabilities in his kit that stem beyond other ADCs, and I think this causes issues with more "binary" champions (for lack of a better word) (
for example).
The previous champions have specific focuses in their kit, along with tradeoffs to counteract said focuses. Varus, for example, has spectacular poking capabilities, which continually increase as a game goes on (assuming he can win his lane). His ultimate is his only form of escape, however, so as the Varus player, you must be wise and ensure not to get caught out. His E has grievous wounds, yes, but it require that he actually lands something in order to inflict said status. His Q is the prime focus of his kit, but can be challenging to land and generally is upgraded first to ensure it's full damage is being received. His W encourages even further for him to utilize a combination of poking with AAs and poking with Qs to maximize damage output.
I feel as if the issue lies within the amount of strategy necessary to execute x amount of damage in older champions versus newer ones. Against Jhin I feel as if they put 50% effort with 100% reward, but versus Varus they seem to put 100% effort for their reward. Why would I want to use a champion that's requires twice as much effort for a similar reward, aside from my own pride?