In short: this is NOT "borderline unplayable". In fact, it's a great mahjong game, I've seen 1v1 ruleset ideas, this is first time I've played it. This game also supports 3P (not usual one though) and 4P gamemodes.
About the game itself: pretty nice riichi-like game with short hentai stories. I've found no real issues. Hentai is mid though, in the 2023 I expect games to have at least some interactive elements. Live2d era fr.
UI is somewhat janky, you can't ignore call button, you must press cancel every time.
upd: Buttons are not in English, but it's easy to understand which one is which. The one you need is always on the left side =) priority is Win (Ron/Tsumo) -> Riichi -> Call (Kan) -> Call (Pon) -> Call (Chi) -> Cancel
About my experience: I had a lot of fun! Played on hard difficulty without cheats, felt like the game is rigged, but... only in my favour =D never before in my life...
For the last boss I've made a challenge for myself, every time I lose, I start from the Stage 1. It was interesting, because that goddess is actually decent in compare to other characters.
In conclusion: didn't jerk off, but experienced some positive emotions. Totally worth it, so, 4/5. Although... of course, it could be better, and I wish it would. There's a big room for improvement. Maybe one day...