Speaking of its ice weakness, isn't the fact that having a double weakness to a common attacking move a big liability for it in terms of switching it in? It makes it hard to switch in on Pokemon with a suspected Ice-type move for fear of suffering heavy damage and preventing it from setting up a sub.
Actually, Ice-type attacks are pretty bloody rare in UU. There really aren't that many viable Ice-type Pokemon in the tier, and even then they are pretty darn rare, which makes that 4x weakness not too much of a problem for Zygarde, though it is one of its more notable flaws.
Smeargle's a terrible example, even given that you're "only" comparing the fact that they need team support. SmearglePass is one of the greatest oddballs to ever come out of Pokemon and has absolutely no equivalent so any comparison is dead on arrival.
I still don't get all these arguments that Zygarde is broken with full team support. Can someone illuminate what makes Zygarde so fucking good that we can look at SD Lucario, CroCune, NP Mega Houndoom, and Calm Mind Mega Alakazam and say "Nope, Zygarde is better than them all"?
If I were to compare Zygarde to any other suspect it would be Smeargle when it comes to showing that a Pokemon that requires team support can still be considered broken. Both of them can reliably lead to a win as long as they have support provided for them. Smeargle will pass its boosts to a Pokemon to win, and Zygarde will win with the boosts by itself. Of course there are differences between them. SmearglePass was much easier to pull off with its formula to win, and Zygarde can have quite a few options to support it in a variety of ways. The fact of the matter is though, if Zygarde is an important factor in reliably winning the majority of games versus most of the tier or make a playstyle a liability, with or without support, it can be considered broken.
I don't know who has been saying full team support, as you really don't need that many Pokemon to support it, and the majority of them you'd have on a team anyway. Though if you want to know what sets aside Zygarde from the other setup sweepers you listed, here is why:
SD Lucario, Mega-Houndoom and Mega-Alakazam are all rather squishy setup sweepers, which can be threatened by rather easily if the opponent chooses to attack, and all of them are rather easily revenge-killed by something faster or priority. Zygarde has a lot more setup opportunities and is threatened by not as many Pokemon as these. To put it quite simply, Zygarde is a bulky setup sweeper, these guys aren't, and so Zyggy finds many more setup opportunities.
In regards to Suicune, this is a better comparison in that they are both bulky setup sweepers. The problem with Suicune is that it is quite easily walled. There are plenty of viable mons that stop CroCune in its tracks: Vaporeon, Toxicroak, Shaymin, CurseGastrodon (<– underrated one) etc. Zygarde on the other hand can hit pretty much everything other than like Togetic, which isn't that common. Once Zygarde gets to about +2 it can be pretty hard to stop. Suicune isn't as threatening.
This all being said, I'm still not sure whether it is broken yet. Zygarde is not outright and obviously broken, though it may be when you look into it.