tl;dr: okay RPGM game that is game first and porn second, ludicrously grindy on normal difficulty
I'm no stranger to RPGM games and RPGs in general, so I'm surprised to be repulsed by Haitenai's grind process. Like other RPGs, progression in Haitenai is primarily gear-driven. You farm drops in each dungeon, run it several more times for excess gear to break down into upgrade materials, then go to the next dungeon and do it all over again. The fact that the developers had to put in respawning chests to guarantee some gear drops should tell you just how atrocious the nominal gear drop rates are. Grinding for rare drops and/or upgrade materials will leave you over-leveled, in turn destroying the game's challenge. When the battles stop being fun, what's the point of continuing?
Speaking of battles, perhaps to double down on the grind-intensive product, the developers also made every mook respawn as soon as you leave the current screen. Most dungeons are 5-7 main floors long, each crawling with ~10 encounters. Select few dungeon spawns can be sneaked past, but most sprites autopath to you even when they haven't seen you yet, and seem to be able to see through obstacles, so you will be accosted by the same few enemies over and over again. Oh, and, if you fight too many battles, the game will drop the MC's stats until you walk back home to have sex. If the game wants me to grind, interrupting it is a grave sin.
I can, however, praise some of the boss fights. They often require some thoughts and tactics that I would expect in mainstream RPGs.
Aside from the grind, my other major gripe is the tonal clash between the game's primarily adult selling point, that being girls freely displaying their genitals at all times and generally accommodating to sexual advances (think free use with extra steps), and the overall tone of the plot. Throughout the adventure, the characters will experience the death of children and their loved ones, all the while the female companions' pussies are still on display. I just don't think drama and arousal are that compatible.
Other assorted complaints:
-the bestiary isn't collapsable by dungeon, so you need to scroll through all 200+ entries to find the beast you want
-the MC acts too much like a beta to my tastes. You'd expect him to be more forward instead of dancing around the bush when he needs to be sexually purified, but Japanese games and tippy-toe MCs are, unfortunately, an entrenched combo
-the porn is just still images with facial expression changes. It's okay, but not especially arousing.
I think Haitenai would be better as a light-hearted RPG with a reworked progression system. Right now, it demands so much time investment from its players (on normal difficulty) but offers so little in return.