RIOT has several things they are trying to balance, and try as they might, it hasn't been easy and I don't think they have conquered it yet.
They are juggling Champions/Items/General Gameplay aspects and trying to balance around all of it.
Sustain gets out of hand? Grevious Wounds. Now sustain isn't so bad, but Crits, add randuins, ninja tabi, and thornmail. Thats just the item side of things.
Tuning one lever often leaves something else out of balance, and it feels like a never ending cycle. Im enjoying the ride, for the most part, but there are certainly a good amount of highs and lows.
To that end though, since itemization affects everything in the game, I think it is the biggest culprit. Just imagine for a moment that there were NO items.
Crits would only be on champions like Tryndamere, Jhin, Fiora and it could be easier to balance. Sustain champs like mundo, and vlad, could be easier to balance. Throw items into the mix and everything is a different story.
I dont think that is a bad thing, the idea of making your playstyle unique and expressing it via items is pretty cool, but its heck to balance.