Well it's all together. Chima was also a reason the Thundercats' reboot ended. So it's not just the Ninjago fans who disliked it.
How was Chima any different? It was basically Ninjago only with animals that walk on two legs, and with drugs of course. And while the sets were good themselves, the concept quickly became old. As repetitive as Ninjago's sets can be (bikes, just bikes), there was always variety in terms of motifs (cyberpunk, jungle, wasteland, etc).
Well I agree, the animation and music were really good, probably the only thing that I would call good about the show. But the direction sucks, with weird editing, odd camera choices and other things (we have to see the beginning of episode 1 twice). The voices in the original dub are just so annoying and exhausting. At the rewatch I actually wanted to see the orignal dub, but after the first episodes I switched to the German dub, who is miles better.
Well yeah, it got better, but only better, and better doesn't mean good. And at the end of the day, most of the story is just not well-written or very entertaining. With bland, annoying and forgetable characters like Laval with little to no character development. Our characters also lacked motivation. The ravens and wolves are on the side of the crocodiles because they are evil, and the gorillas and eagles are on the side of the lions because they are good. Same goes to the later factions. The tone was also inconsistent (Cragger's henchmen being silly in the background of a battle). And I don't even want to start talking about comedy, because at best it's as bad as Ninjago's comedy.
Season one is just plain boring, repetitive and lame (the racing episodes, gosh). With the second, they didn't seem to know what to do with it. And while season three is better, it's mediocre average at best. Chima is not even bad in the sense of "it's so bad, it's good" bad, like the Donkey Kong Country Cartoon, Power Rangers or Batman with Adam West. But Chima is so bad, that it's just bad.
So Chima is hated because of: bad writing, bad characters, bad voice acting, repetitive set motifs that don't appeal to everyone and the killing of Ninjago's and Thundercats at the time.
Also, Nexo Knights was originally supposed to have a final season, but that one has been canceled. If I'm not mistaken, the story was told to the end in the app game.