It's a subtle point but it's definitely Mania's key weakness. Sonic 3 is very much designed with Knuckles as hard mode. Knuckles even has slightly harder bosses than Sonic does. I think a lot of fans forget that Knuckles' alternate paths are often harder than Sonic's main path. It wasn't just more paths to take but a more difficult run. I think this aspect of Knuckles' story mode is forgotten because a lot of fans who play the Classics keep promoting alternate paths without much thought beyond replay value. I think Mania unfortunately falls into this trap. Knuckles doesn't have many alternate paths and the ones which he does have only really show his abilities instead of being challenging. The only times Mania does do so is when it's copying the design of 3K.