Let's be honest here, the hate is primarily coming from Zelda fans, they know that Zelda becoming an open world ARPG means that it will be compared directly against Horizon.
As a person who's massively bought into BotW hype, this is very true. But it's coming from both sides.
The two games both often show off a bow wielding protagonist, riding through some open world terrain, fancily taking down robot enemies. Both games have had their respective fans gush about how it well it captures the feeling of Mononoke, or in BotW's case, sometimes just being called Miyazaki-esque. There's a lot of parallels
on paper, and that's just enough to incite a fan war.
It's a real shame because I personally expect the two will complement each other well. The narrative is clearly different, and they don't look remotely similar aesthetically. On top of that you can guarantee that whatever gameplay, locations, etc Nintendo has come up with will not feel like what GG is delivering.
