Good game, I have a few thoughts, I'm surprised so many posts here praise the story/plot when it's all been done before, and I think there are lots of h games with more interesting stories. It develops a tremendous atmosphere, but fails to do anything novel with that.
Many of the characters aren't really very interesting, which is too bad as compared with MY]R. In that game the character motivations are a mystery and turns out that a lot of the motivation is collecting genetic samples from another world, which then has a mutiny on the expedition. While in this game, the good guys are good, the bad guys are bad and it's all vanilla. The demon king has some funny dialogue and its clever that she thinks you're an assassin, and its hinted that there could be war, but no action is taken that would push the characters out of their 1D. Some characters are interesting though, I like blue Oni, she has realistic conflict. Furface is well done. Urashima has interesting backstory, Oni king has interesting backstory. Mechano is underdeveloped, and not enough is done with his setup that he will transform into a human. Guide doesn't act real either, she doesn't 'guide' you enough, once you go through the trials and talk to death she should clearly tell you what she wants you to do to save the world, after all its her world you're saving, not a game to her. In End 2 Guide says that you weren't the first brought here by her to save the world. She wanted to pretend it's a game to make the transition easier for you.
The 2nd ending is the best in terms of story/plot because you experience the max of the game world in that ending. However there should have been more content regarding the seabed girl to tie it all together, she needed some actual dialog. How should you know to bring her scales? In my play through I had already acquired them before I talked to her so she just took them. I think she should nudge you in the 'evil' direction by asking you for stuff after fucking her, which slowly starts to corrupt you. And there should be some dialogue before you fight to kill a dragon "If you kill me then you are truly lost Hero." Also the spirits shouldn't fight their patron dragon. The endings all feel a bit rushed, and the 2nd has the largest scope and feels the most rushed, why can't you go into the towns and see what people think of the corrupted hero?
The game actually functions as if you playing a character playing a game. The real world stuff has no impact on the game and vice versa (aside from a few h scenes, that have no impact on the story). The story of the fantasy world part is pretty unoriginal, and boils down to normal tropes: hero wakes up in a new world, is guided through trials to become stronger, will ally with the 4 elements, and finally fight the demon hoard that is trying to invade the world. There's a lot of unrealized potential for some really interesting stories here, but they went with the simplest one.
What if you were brought as a conduit and you accidentally open some barrier to the real world for those aliens to attack your world? What if the Onis were actually looking for some powerful 'evil' artifact to help them in the war vs the demons? What if this was a plan from the start for Death and Guide to develop a hero strong enough to kill the zodiac so they could ascend as the ultimate beings? What if the seabed girl is some lost alien and the aliens are just looking to recover their princess? What if you are in the real world walking back to the train station with all the other Joes and then out of the corner of your eye you see a black form moving over near that storage shack? (I was always expecting this)
Even with some faults the game is undeniably badass, it peaked for me when I first entered the illusory world, but getting your flying house was also super badass, I think the underwater could be similarly cool if it was designed better and more clear to navigate with more content. Gameplay-wise the game was strongest before the battles started getting tough. I'm not sure how the enemy scaling works but I'm pretty sure there's some kind of scaling going on, because I did try to grind a lot (easiest way I found was in the illusory realm) and it doesn't make the game much easier. The game isn't particularly tough if you have a good strategy but the difficulty curve makes it feel bad once the game goes 'open-world', once you get all 4 spirits with upgrades you think that you will be very strong, but fights are still tedious. In END 2 the best way to win is just defending and getting huge counterattacks which is cheesy.
Some other loose ends: if you kill furface on the island you get claws, which have no use. White Oni has no conclusion, and her affliction is never explained in more detail. I'm guessing its different from that of the Oni King. Wish you could mate all the cats, wouldn't even take much work updaing the one cat cg.
It has tons of h content and you can really see the artistic vision and effort which definitely puts this game above a lot of others. It's too bad the girls only have one body type though, and each encounter is almost the same, where you essentially rape the girl and she ends up loving it.