Zhonya's has always been in a weird position in my opinion and Stopwatch has only made things more weird.
For me, Zhonya's has always been a bit of a 'gamble item' so to speak. The defensive stats are a bit nice early but don't mean much late and the actual active effect can be gimmicky if we're all gonna be honest here. There are times where you can use it to make plays, times where you can use it defensively and your team can save you, and then times where you use it defensively but realistically are just prolonging your death by a few seconds.
With the introduction of Stopwatch to the game, Zhonya's in retrospect seems stronger. The ability to completely shut down early plays with Stopwatch is insanely powerful, don't get me wrong, but then we hit the problem of reality verse the problem we think is a lot larger then it is. In pro play, Stopwatch was disgustingly useful to the point where it influenced the solo q meta for awhile. Stopwatch's rune gets nerfed and then nerfed again later on to where it's viable but no one takes the rune cause there are better choices. Stopwatch gets slightly buffed and we're back to the same middle ground where Zhonya's is in my opinion.
Overall, I think in the right hands Zhonya's can insanely powerful. With coordination and proper communication, Zhyona's/Stopwatch can actually completely change the tides of a teamfight.
Realistically though, I think Zhonya's will always be a stable item in the game but will always be semi-gimmicky too. It's great in the hands of the right players but a waste of an item on some players. In higher elo the item can extreme in its usefulness because those players have such a good understanding on how these Zhonya's situations can/will go down, whereas in lower elo the item is almost equally as useful but even more useless in the sense of the players who can use it right but also in the sense of the players who don't know how to react to a Zhonya's in a teamfight too.
Holy moly, how did I end up typing this much on such a simple item? Good morning world, I, am, awake!
Kind of, partly; not really but getting there.