As a preface, if you enjoy platforming above all else then you might develop a drastically different opinion of this game than me. That said, let's begin with the review of everything that went wrong with this game. Breaking it down into sections, let's start with the obvious major issue that plagues every other aspect of the game by osmosis: Art.
Art (and perhaps more importantly Art direction) in this game is really noticeably amateurish, and not just in a charming indie way. More in a "Why would they have a foreground that can literally obscure the player, enemies, and projectiles?" way. Why would the lava slimes be the exact same color as the lava in the background, and when they die they shoot lava that's visually cluttered by the explosion of loot upon death? This might sound like an isolated issue, but over half the enemies in the game you'll encounter (maybe more?) are variants of exploding slime enemies. All of them are color coded to blend into the background of where they are and often they have explosion effects that make me wonder if I'm old enough to need reading glasses. These mistakes are fundamental issues that someone in the team should've noticed just in the course of seeing that purple enemies on purple surfaces on purple backgrounds that shoot purple attacks maybe isn't a great idea. Good difficulty comes from overcoming *clear* hazards and *visible* attacks. There are spikes in the foreground, spikes in the background, and spikes that actually kill you. Why are all these present on the same screen with the same art style? Why do many enemy attacks have much bigger hitboxes than the actual animated attack? This is really basic stuff, so I'm hoping the devs already learned their lesson and didn't repeat this kind of mistake in their next game(s).
Similarly to the overall art direction, the menu design might be the worst I've ever seen in a game. I'm not even sure where to begin on how bad it is so I'm not going to go on about it. Suffice to say, this is literally the most basic thing a game should have. The map clarity resembles your recollection of a night you were blackout drunk, even when you buy the maps from the guy who is only there to sell maps because the devs played hollow knight and just decided to copy things without considering whether it actually made to sense to add them to their game. On that note, the death penalty has no reason to be in the game at all. More on that later.
And before the inevitable refrain arrives, yes I beat the game on Hard so this isn't a "get good" issue. On that topic, the difficulty of the game is all over the place. Combat in the game is, frankly, extremely bad. Moreover, enemies are designed more to waste your time than be a genuine threat. And even if they were, the checkpoint system completely removes difficulty from combat outside of boss fights. You're actually better off killing yourself if your health is particularly low because you're probably 5 seconds from a checkpoint. This makes the death "penalty" a meaningless souls/hollow knight reference that doesn't actually affect your gameplay at all. What's really humorous is they must have thought it was some kind of huge deal since the payment to get your soul back if you don't want to go get it in the rare occasion you didn't get it back immediately is losing ALL of your exp. This feels needlessly punitive to people who aren't good enough at platforming or combat that this mechanic actually affects them, while meaningless to anyone just cruising through and killing themselves at opportune times near checkpoints.
Anyway, most enemies are either the aforementioned exploding slimes who are designed to literally do nothing (they don't attack) but waste your time upon their death explosion that will force you to jump around or shield, or enemies that pose no real threat to you until the enemies in the very last area of the game. More importantly than the enemies themselves though, the actual feedback of combat is abysmal. There's no real hit feedback or hitstop (except when you yourself are hit for some reason), when you block something with the shield there's no indication you blocked (which can cause situations where you're not sure if you actually even blocked a hit), there's no castlevania backdash cancel, no jump cancel except out of dash (which can't be cancelled into), and basically no fighting game or castlevania-esque cancels you can perform that make combat smoother and more responsive. This makes the game very extremely basic in combat despite the wide array of weapons and tools. Ultimately they're a series of bandaids over a rotten core. Give me a whip and 6 different ways to move cleanly over 6 different weapons and clunky movement with no cancels. This leads me to my next points...
Bosses. Because this game is always platforming-focused above all else, especially in the boss fights, this might be an area where I can admit the game doesn't have fundamental game design flaws but rather just isn't to my personal taste. If you want boss fights to be platforming extravaganzas then I think you could really enjoy the fights here. Unfortunately for me, once I'd seen 100 teleport arrows puzzles in a row, I wasn't exactly thrilled that the boss fight(s) were also a teleport arrow puzzle. In theory, a game like this could break up all the platforming with boss fights being more focused on the combat system. However, since the combat is so barebones it can only really be a platforming exercise with occasional hitting the boss with your weapons of choice. The final boss especially felt like some kind of self-parody of the game, wherein the answer to every boss mechanic was "Just teleport arrow, bro." and the answer was correct.
If you've read this far, I'm sorry you've had to suffer through this much of my prose. However, I must include one final problem that plagues the game. Backtracking. So, the fast travel system is very poorly realized to begin with (not to mention how annoying it is to to have to go through the imprecise selection process on the map) but in addition there's no way to get back to the fast travel points but by traversing or by using a consumable you will probably only have one of at a time for most of the game and costs money to buy. What this means is, in conjunction with the devs never thinking of the concept of a "shortcut after puzzle completion," that whenever you do the latest teleport arrow platforming puzzle, you gotta do the same puzzle backwards just to get back to where you started. It'll often be basically the same thing the way back, so it's not "new" or "exciting" gameplay to have to go back the way you came. It's just boring and made me want to uninstall due to how annoying it was every time. People say this game has a lot of content, but frankly I think if you actually had convenient fast travel and didn't have to do every puzzle on the way in and on the way out, I could've cut like 10 hours off my finished playthrough on hard.
Finally, I'm genuinely happy to be done with this game and I hope reading this has been as cathartic for you as writing it was for me. Good luck to the devs on their future projects and I hope they learn from their mistakes.