Urgh, you're asking for a pretty hefty summarization there, but I'll give it a shot with what I can remember off the top of my head.
Firstly, and this is my personal pet peeve, the game's writers are constantly dictating what the player character is thinking or feeling. The player doesn't have much control or voice, and no matter who or what you have shaped yourself to be in the game, you will be depicted as whatever Savin and his out of touch writing team need- or just want- to depict you as.
Which leads into the second major problem, some of the writing team just hates the playerbase and isn't even remotely discrete about it. They pick fights with people on the forums or over Discord, and when writing for the game, they make a point of depicting the player character as spineless, shameful, and pathetic at every opportunity. The most egregious example that comes to most folk's minds would be Tobs- short for The Observer- and his kitsune content.
Over some petty back and forth about incest fetishism with some rando on the forums who asked about whether or not Kinu would ever be a romance option, Tobs decided to write an expansion in which Kinu finds love with Tobs' self insert who 'is better at everything than the player character'. The entirety of the kitsune expansion is just packed with venom and spite for the playerbase, taking every chance he could to belittle and disempower the player character. He then went on to commission lewd fanart of his self insert having sex with Kinu, and wrote fanfics about their sex life. He's a weird and petty guy, that Tobs.
Thirdly, the lore is an absolute mess right now. There are so many plotholes, and so much of the writing is spectacle over substance. Savin and his team write by the rule of cool, they aren't really thinking longterm, they don't have answers for or solutions to plotholes, and when they're really being pressed by the players to explain they'll just snap back with some variation of "I am the writer, whatever I say is canon IS the canon."
To offer some examples, the living gods in the game are depicted as being all powerful super beings that can show up at a moment's notice and down unbeatable foes like it's nothing. In theory, they could just solve the entire game's plot in a heartbeat. Your adventure is pointless if the world can be saved with a sneeze from these super heroes- but the only explanations given for why they aren't actually bothering to save the day is 'they're busy', or 'they're watching Kass to learn from her'. Buuuut, these living gods can also put souls back into bodies thereby nullifying corruption and could effortlessly fulfill Kass' wish (the exact thing Kass is studying so why are you even watching her if you already know how to do what she's struggling to learn to do), and just.. Y'know.. Stop this whole demonic invasion dead in it's tracks. Trade her for whatever knowledge they're hoping to get out of her. Again, they just don't. There's a legion of justice comprised entirely of different versions of SuperMan, and they're not doing anything. They aren't the only ones either, the writers keep introducing OCs and self inserts that could effortlessly save the day, and they just aren't. They're writing the player to be a pushover who can only gape in awe at the magnificence of the rippling muscles and throbbing horse cocks of these TRUE champions that the writers assure us are so amazingly interesting and cool and hot- and it's just a miserable slog that keeps piling on questions like "What am I even doing here, why isn't this guy saving the day if he could do it so effortlessly?"
Then there's questions about why everyone is so cool with the Mallach temple grooming kids to join their cult, or why all the living god cults are so accepted everywhere when the living gods themselves are so apathetic and aloof that they can't even be bothered to care when their followers are dying to demons right in front of their altars. Or why everyone in this totally pure, non-corrupt world is even sluttier and more shameless than Kass and her demons are. Or why you, the player character, should even care when nearly everyone else in the plot is just shrugging their shoulders indifferently to the demonic plague spreading through the land. I mean, if they don't care, whatever, you're not even from the north in most character backstories, just go back home to the south and leave these apathetic demigods to find the motivation to fix everything with a snap of their fingers.
Fourth, the writing itself is just bad. The different writers do not have a unified voice, they all have very different writing styles and approaches to every scene, and depict the player character as very different stereotypes with the only unifying theme being that the player character is weak and falls for any tits or dicks above a certain size, love at first sight if it's a horse dick. Dialogue with characters is usually clumsy lore dumps. You ask someone why they aren't eating the peas on their plate, and they'll start telling you a three page story about how their father died to a pod of peas with little stick arms and legs when they were only six years old, and all throughout the lore dump, you're a vapid vehicle for the writer to show off their shiny new character. You sit there and emote exactly how the writer wants you to. "Wow, that's heavy stuff. You are filled with a deep sense of empathy, and feel ashamed for having unwittingly brought up their pea-related trauma so thoughtlessly. But you are too socially inept to come up with any way to make it up to her, so you sit in awkward silence and allow her to continue monologuing about her life."
Fifth, this game doesn't do variety. Horse cocks, horse cocks, futa horse cocks, men with horse cocks, Cait's taking it up the ass from Brint and the boys again, penis goes into vagoo, woowoo. Savin and his team have like three collective fetishes between them, and that's all they write. Big boobs, big dicks, and muscles. Unless you're someone that can get unreasonably aroused over seeing your seventeenth horse dick within the span of thirty minutes of playtime, you'll find yourself entirely flaccid/dry. Nothing kinky happens in Hawkethorne. It's just meat on meat forever and always.
Sixth, this game doesn't do romance. Relationships are awful, you can't expect anyone to be exclusive or faithful to you, Brint or Arona will always show up to have a piece of whatever you're getting into. Cait too, she'll sleep with anyone- even the people she claims she absolutely hates like the demons, if you give her a chance to gag on a demonic centaur's cock, she's down for that and will swallow as much corrupt spunk as she can get. You can't get to know anyone- you can get them to give you their life story in a lore dump, but actually building any kind of attachment or bond between you and them is just not something the writers comprehend how to do. Kinu is probably the closest they ever came, I think? But she's a trap, because down that way, only Tobs lay.
The game is selling itself as something it's not. It doesn't give players choices beyond what they look like- and even that's been getting pruned back as much as possible because Savin finds it to be a hassle they don't want to have to deal with any longer. The gameplay is really clunky, awkward, and not at all interesting, anyone playing this for the combat has... Interesting tastes, would be the nice way to put it. The exploration is pointless because the maps are large but there's nothing to be found across any of them. The game has been in development hell for ages now.
People are upset because they wanted it to be good. Character creation is rare in adult games, and there are options to play as things that you can't normally play as in ANY games, period. It's just that this game isn't good, and it isn't letting players roleplay, it's dictating everything they do or say to them without bothering to ask for the player's input or opinion. It's disappointment and frustration. We wanted better than Savin and his gang's OC VN.
