Personally, I didn't enjoy this game after a couple of hours. I think the fandom is making this game overrated. Sure, it's impressive that only 2 people made this AND hand drew everything you see in the game. It's very impressive. The gameplay, however, wasn't (and not exactly because the gameplay was difficult, but WHY it was difficult). It can be fun when you first pick up the game, but not so much after a while. It gets dull. Very dull. I think I can see what the creators aimed for to make the gameplay, but they didn't execute it exactly right. The level design of Cuphead is extremely simple: Dodge thousands of enemies while holding down a button.
Instead of the game developers making unique gameplay to keep the game interesting, they made more enemies appear. That's why the game is difficult. You jump over the 40 enemies and their projectiles that all show up at the screen at once. It's not difficult because it's hard to master, it's just unfair. Fans will say "that's what it's supposed to be like. You're just not good at it." If being good at a game is the key to making it fun, then I won't play it. Games like Geometry Dash do a great job with that type of playstyle though, because it seems like it's your fault when you die. You can notice what you need to improve on. For Cuphead, the only thing you can improve on is dealing with the rage of an enemy spam.
The bosses may seem different from each other, but their really not. The only difference is how many projectiles they spit at you. It gets boring progressing through the same thing over and over, especially when those things are random and too chaotic.
Cuphead game devs, If you're going to make something that requires a lot of hard work in art, then make a show (which they have) or something along the lines of that. Not a video game that repeats the same un-enjoyable thing over and over. If the game was shorter, I would like it more. I would enjoy it more if they made new gameplay as you progressed, too. I'm not talking about how the different charms you can buy can change what holding down a button does. That's not making new gameplay. It's like putting on a different shirt. You're the same exact person with that shirt on. You just look a little different.
I'm making this review WAY too long, so this is the last this I'll make you read: This is my opinion on the game. Most people like this game, and if you're looking to play it, you might too. Just don't be surprised when you find yourself repeating the same type of level. I'd suggest something like Metroid Dread or Hollow Knight instead.
(To any upset fan who thinks I'm giving this 1 star because I thought it was too hard. I'm not. I'm good at the game. I still didn't enjoy it :D)