This item is super good for what it offers. It has a fantastic build path with 
. It is one 2 of the top AP items. It has the best defensive active in the game. It also is highly cost effective in stats regardless of the passive. Not only that but the AD equivilent which would be 
isnt cost effective in stats and has a shitty passive in comparison.
In regards to comparing Mercurial to Zhonya's; AP does not have Crit. Inherently, AD scales stronger because of the existence of Crit.
Not only that but the AD equivilent which would be
The AD equivalent to Zhonya's is more 
and everything that builds out of it.
Zhonya's basic job is allowing squishy AP champions a burst of survivability so they can burst, get a burst of defense, then burst some more within a team-fight setting.
Life steal allows sustained damage champions the sustain to sustain through a fight while dealing sustained damage.
Life steal also plays the major strength of sustained damage (extended duels) where Zhonya's plays to the strength of burst (team fights) giving both AP and AD glass-cannons an interesting defensive capacity that allows them some degree of durability if they play there cards right, keeping the desire to build tank items on glass-cannons at bay. Which in turn lets glass-cannons be glass cannons, and allows items to play a much bigger roll in champion identity rather than being boiled down to the same basic mix of offense and defense your typical RPG suffers from.
I'd also like to point out spell vamp is about as terrible as Scimitar. Life steal is more in line with the overall reasons and application (as well as WTF moments and complaints) as Zhonya's.
TLDR: Zhonya's counterpart is life steal.
Hourglass has been and forever will be a brokenly OP item as long as it has the second highest AP offered and the best defensive active in the game. It has literally no down sides since you can get it as a first item and still do a lot more damage from it and also get to escape a lot of bad situations. If it had say 80 AP and a cost of 2900 it would be more justified.
In dota 2 of you want the active you get nowhere near the offensive stats and no defensive stats. It is a support item that is common on some heros and basically never bought by others. Eul's is far more balanced then hourglass.
Because Mages aren't even allowed a full offense item
Honestly im with you. zhonyas makes up for far too many mistakes made by mages. there is 0 counterplay aside from hard CC (which counters everything, aside from 
}. Either reduce the AP it gives or make the stasis more like 1 second or 1.5 seconds. It should be used skillfully not. oh i dived and killed the enemy but im gunna die zhonyas. Zhonyas also hard counters many abilities. way more than any other item/ability. Heres a list of things i can see zhonyas being "saved for"
Amumu ult Annie ult Ashe ult Blitz combo Brand ult (made useless) Caitlyn ult Cass ult Darius ult Draven ult Ezreal ult Fiddle ult Fiora ult Fizz ult Graves ult Gragas ult Hecarim ult Jinx ult karthus ult katarina ult Lux ult Malphite ult MF ult Morgana ult Nami ult Nautilus ult (poor naut :( Nocturne ult Nunu ult Orianna ult Rengar ult (makes him suicide) Riven ult (though i thoroughly support everything that counters riven) Sejuani ult Sion ult Sona ult Syndra ult Varus ult Vel'koz ult Vi ult (makes her suicide) Vlad ult Zac combo Zed ult Ziggs ult
That is just ultimates for the most part. Now for champions that easily abuse zhonyas.
Heimerdinger Katarina LeBlanc (she honestly needs ANOTHER defensive option? Shes got her passive, and W and E!!!) Zed and Wukong (if your playing around) Possibly Zyra
Now i also recognize there are champions who basically require this item to be relevant.
Fiddlesticks karthus Kennen Lissandra Morgana
Zhonyas also does not promote teamwork. Nor does it pay child support. But the whole point of this is to show that 1 ITEM is far too powerful.
