User blog comment:YourPrivateNightmare/Hail the hourglass!/@comment-15244341-20130915194443

Bad answer: GA is better if you are lazy. ADCs are really micro intensive, specially if you are actually trying to kite the bruiser/assasin/whatever. I know I have my hands full when I play ADC, but I'm pretty mediocre at it so...

The actual problem is, I think, that ADCs with a GA are both a target you need to focus because of the DPS they output and a target you don't want to focus because you will most likely die after bursting them down for actually no gain (apart from taking their ADC out for 5 secs, I guess). Zhonya's, however, only takes the focus away for a few seconds. That, in my opinion, makes it better against assassins but worse against bruisers (which normally kill in a bit more than a single of their rotations). So maybe that's it. Also, you might make someone waste their whole burst with GA but not with Zhonya. That is what makes it better in my opinion.

And a last bad answer: GA is cheaper xD

So it may be okay, just not ideal.