A high amount of people say Zilean's ult is very strong and should be changed for his rework. I see no reason why his ult should be changed because you cant counter his ult in many ways. I do understand the cooldown is very short with his W. 
Zilean's ult has an inbuilt weakness. Several, actually.
If he's ulting a teammate, he's not able to resurrect himself (duh.) Zilean is super fucking squishy, and his ult is fairly obvious and doesn't last very long. If Zilean doesn't ult himself, he can be bursted down to prevent him from aiding his team.
If Zilean DOES ult himself, he runs into the problem of not being a hard carry: Zilean can't solo an enemy team unless he's MASSIVELY fed and is building full AP. So, if he's ulting himself he needs to make use of that (by being a meatshield and the like.)
His ult is easy to counter: If you know the enemy team has Zilean, watch for the ult animation. Target everyone but the person with the ult (ideally you drop them very low, then leave them alone till the ult runs out, then murder them.) Additionally, since Zilean is so squishy it is feasible to target him first, thus either forcing him to burn his ult on himself or die.
His ult is one of the best designed abilities in League, it promotes a lot of strategy and tactical play. I would be GREATLY disappointed if they removed it, as the 'Can't Killean the Zilean' video on youtube would no longer give me tears of laughter, but of sadness. Highly recommend watching that video, BTW.
It's less OP, than it is completely frustrating.
People also say that lag is the reason they died, don't believe everything you hear.
I love zilean. He is one of my favourite supports and I have first hand experienced that his ult and everything about him is far from op. All you have to do is wait 7 seconds. Then its free game on whoever you want to kill. That's what they did in LCS. The cooldown on his w is 10 seconds at level 1 which is almost an eternity, at level five its around 5 seconds. The only reason people say he is op is because they dont understand his kit nor how to play against him. Pro Tip dont group up and he wont have much power.
its not even the cooldown that's the biggest issue imo. it's the duration of the ult. he plops it on someone and yo get the visual cue so you don't attack them. and you wait, and wait, and wait, and wait, and it NEVER goes away. in most cases its actually better to kill the person ANYWAYS just because it will stop them from doing damage while they respawn!
I think it's OP but I have bigger issues with other champions and would rather see changes to champions like Yi or Kat where a single person with very little skill can carry just using one of them because they are OP over a champion like Zilean.
I feel like his ult is more cheap than anything and very frustrating and maybe that is why people hate it so much. I always play in teamspeak and when we play against a Zilean who uses his ult I rarely hear "That's OP" over "That's just BS" which is more a complaint of frustration to me. Often working your butt off to kill someone only to have them rezzed.
I think when he does it to someone else over himself this creates more of a frustration than anything as at least in my games no one goes after him, as he is seen as a waste of energy. But I do think that this is more a trend of some ults having "too short" a cooldown and others having "too long" a cooldown. I feel like sometimes when I play I get frustrated often when someone repeatedly ults and I have to wait a long time for mine even with a lot of CD reduction.
But those are just balance issues and this game has a lot of those.
His ulti is OP but his OPness is balanced by the loooong CD(and in the current kit a whole spell is basically sacrificed to reduce the Cd of 2 other spells, so the trade-off is big) and is his signature move so it should stay as it is(of course, adjusted CD and mana costs when his kit is reworked and things like his W are probably reworked)
Zilean's ult is a bit difficult to use, and you have to use it to its fullest extent to even play zilean at all. Those bombs are his only source of damage, he has no healing abilities, and quite honestly his ult is the only reason he's fun to play. A rework could be nice, but I'm not sure how his ult would fit into a rework. I'd be willing to sacrifice that for a more balanced kit, but I'd sure miss rushing GA in Ascension just to be an ***hole. At least I wouldn't be standing around and throwing own speed boosts that nobody even notices....
His ultimate is way over powered but it is made up for because the rest of his kit is entirely garbage.
I'm saying yes, but not because its op or up, its just because its so boring. The time mage concept has a lot of possibilities, limiting to a free GA isn't its full potential.
I think Zilean's ult is only OP in ARAM since its a do or die type of game mode with no recall,so a free revive for a teammate leads to an easy win since theres no GA in that mode. On SR though,no its not OP.
