Why isn't Zhonyas considered op?

Why isn't Mecurial Scimitar considered OP? It also ties for the maximum damage in it's damage type, gives defence against the other type of damage, and gives you away to get out of Zed's ult, Fizz's Ult, Zilian Bombs, Hooks and Grabs, etc. Plus it makes you move faster so you can actually get out of the nasty situation.

The last one is the kicker. Yeah, Zhonya's makes you untargetable, but it's easier to be calapsed on afterwards. That's it's weakness. Mecurial Scimitar has the exact oposite weakness.

Because most of the people who use it don't use it correctly and screw themselves over. Is there anything funnier than a Heimer using Zhonya's while he's surrounded by the enemy team?

It's complained about every so often, usually by people who play Zed. It's just a class difference, allows APs to wait for CDs and not get murdered when because the other guy can auto while he waits. It annoys me when I go against it, but I don't honestly think it's broken.

zhonyas is actually considered broken but the reason it isnt spoken about very often is because people who main ap like to bring on their smurfs to downvote and backtalk anyone who speaks the truth about it so they can spam it on their fizz and leblanc so they get freelo mid while zed players get the d by the adc and mid itemization

Zhonya's isn't OP because of many reasons. Usually casters in mid lane get it, so if they themselves are against a mage and rush it, it offers them no defensive stats (MR) to help them mitigate damage from their lane opponent. Also its active is a double edged sword. If you do use it to dodge damage and force the enemy team to waste resources (i.e. summoner spells, ultimates) on you then its incredibly strong, but if you are behind, activating the item only allows for the rest of their team to collapse on you and camp on top of you until you come out of it and get killed by them.