What is so bad about Lost World? (The only other recent console Sonic I never played was Black Knight so I'm well versed.)
The game's controls, while far from terrible, felt a bit too scripted when it comes to parkour. For example: it doesn't feel like you have enough control over Sonic's wall run. That being said, even with those controls, the first half of the game was still pretty good. There's generally no need to use the wall run much unless you want to find red rings or go through a certain alternate path, but even then it's not really game breaking. Just somewhat uncomfortable to use.
The main problem though is more the design of about half of the levels, particularly in the second half of the game. The second half has a lot more gimmicky levels that aren't really all that fun and also don't mesh well with the more normal stages in the game. It also re-uses assets from previous worlds in the second half and it gets frustratingly hard. It feels like while the game is polished, they didn't have enough time to properly design half of the game. Making it a game that hits just as much as it misses and feels unfocused.
There's also other stuff like the bosses being too easy, not being able to replay stages without having to go against the stage's boss, it being an entirely different game from Colors and Generations which means not everyone who liked those games would enjoy Lost World. Also the story, while not as bad as in Sonic 2006 or Shadow the Hedgehog, was kinda bad in comparison to Colors and Generations. Both of which had stories that didn't amount to much more than jokes that are drawn out so much that they're just plain unfunny.
By the way, one of the people who got fired was Christian Senn, who also happened to be involved with Sonic X-Treme. Another failed attempt by a western studio to make a regular Sonic game, except that one was never officially released. The development behind that game was seemingly worse than for Sonic Boom though, as it involved people almost dying because of stress.
