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The hate for Unova is quite literally only from crybabies who have never played the game, who complain as contrarians that Gen 5 is considered the best gen by the real fans, after which everything else went to dogshit.
The people hating on Unova always quote easily searchable things like lower than expected sales and some Pokemon being ugly or bad copies of Kanto. In short, subjective opinions and clickbait articles.
It's not like Gen 5 is perfect by any means, and it facts it has quite a few problems. The game is way too talky, and you are cockblocked by basically everything and everyone to progress the story or a character development. Despite there being good content in this writing, it gets far too lenghty and it stops your gameplay far too often. Something easily fixable if Game Freak could find out how to create a skip button.
The trainers are also far too linear. Every trainer follows the Pokedex to an A, using exactly what you would expect to find out in that route and in the next Pokedex blank spot.
Trainers also started to have no more than 3 Pokemon for whatever reason, and they always follow the same pattern. If you are supposed to be around level 30, a trainer with 3 Pokemon will have 3 level 28 Pokemon, a trainer with 2 Pokemon will have two level 29 Pokemon, a trainer with 1 Pokemon will have a single level 30 Pokemon. It gets straight up boring to fight trainers when you know exacly what to expect, and you are doing nothing more than fighting the wild Pokemon of that route that you cannot run away from. Hard mode in the sequel only fixes this for major trainers, and even then the Hard mode unlock requirements are absurd.
Other Pokemon also suffer from very strange decisions like weird and limited movepools and completely unacceptable evolution levels.
There is a few more problems too I might not remember at the moment, but I doubt that any Unova hater knows even a single one because they clearly don't even play the games.