we try to have VFX, sounds and other indicators be as clear as possible. We want people to master the game itself, not odd UI and noisy graphics.
When I first played Draven, I've faced a Malzahar and noticed it's more or less impossible to tell where my axes are going to fall underneath all the graphics Malzahar sends all over the place, especially his pool.
Two days ago I played Tryndamere, and while I was not at peak capacity physically, I had a hard time telling where I am in team fights. We're talking Arcade Sona ult, Elise Rappel, Graves ult, Maokai ult, MF ult, Sona ult, and that Christmas Veigar skin all in one place. It's a good thing I don't suffer from seizures >.<
I did notice with some of the newer skins they certainly do "pop" a lot more, and it feels great, when they're alone, but with some of them it gets to the point they are overbearing, or just add too much visual noise and clutter:
Tyrant Swain is awesome, it's just shy of being too much.
Blackthorn Morgana is over-the-top. Everything about it is too loud.
Slay Belle Katarina, I like, but the green leafy effects added say to her W just bring some things over the comfort line, especially in team-fights.
The list goes on, but these are some examples. Blackthorn Morgana makes my head hurt.