Honestly, the only real problem I have with assassins is that they can brute force their kills on healthy targets at all stages of the game. It used to be that, aside from some outliers (looking at you Leblanc, Talon), assassins in a vaccuum used to have to poke you down and go in with their full combo after you're weakened. Would be great if assassins were more focused on missing hp type scalings that are quite terrible on fully healthy champs or modifiers based on whether the champ is disabled or not (and whether it's soft or hard disable). I would like them more if they played more like fragile tricksters than brutes. I can respect a well played early game shaco that successfully reaches his peak, for example.
Another gripe I have is that itemization has gotten more offensive overall. Banshee's veil and and guardian angel were once purely defensively stated. In addition, defensive items like thornmail worked differently, scaling with the damage enemies did and making it an overall more dangerous item. I always thought it was fair because the reflect is magic damage, so counterplay was built in (nowadays, that would be a much easier thing to counter also, as qss builds into an ad/lifesteal item and maw grants lifesteal and spell vamp upon lifeline activation).