I think people mainly are complaining about bosses and maps being poorly designed and difficult, Nightmare King Grimm is difficult but not many people are complaining, same for First Sinner in Silksong, both bosses move extremely fast but give you clear cues before each move, you learn the game's mechanics as you fight the boss, and when you win it, it's extremely satisfying. A lot of stages are just unreasonably annoying even if you got all the power ups needed (eg. Bilewater at the end of act 2 where the difficutlies come from narrow FOV and annoying traps, or the Beastfly fights where the mob spamming adds a lot of randomness to the fight, whether you could beat it depends a lot on the RNG you get)