This has all been said in other threads, and I'm not sure why this hasn't been merged. But, Zay has issues with dropped passes, has poor hands, doesn't highpoint the ball, doesn't contest the ball, has poor situational awareness (doesn't see the defenders around him, and adjust accordingly), and has a woefully limited catch radius.
The best thing that can be said about him is that he's versatile, and has a comprehensive route tree, But, for all his versatility, he is mediocre, at best, at all of it, with the possible exception of blocking.
And, I don't really hear anyone crowing about his leadership, either.
In the draft, he was sold as being "the most NFL-ready" WR in his class, with super reliable hands. That turned out to not be true.
And, personally, I look at his play, and his progression through his time in Buffalo, and I suspect that he very well might be about at his ceiling.
All of this adds up to someone who does not offer enough, and who will make costly mistakes.
And, compound that with the frustration that some feel is a wasted opportunity having Duke Williams on the PS, (I am included in that crowd) who could fill Zay's role without so many limitations, and you can understand why so many want him gone.