(Background : I have been studying Japanese for years, first in high-school, then as a self-learner with not enough time because of my official studies, then work; I have a N2 passive comprehension level, read novels, watch movies/animes without subtitles, know about 85% of Joyou kanjis through Anki, but have a much lower N4/N3 active production level. This year, I decided to actually get serious again and study to strengthen what I have acquired through native media consumption)
Actually, the more I use it, the more I like it. I have a lifetime subscription to Bunpro/Wanikani and MaruMori. I got this one last, when they had a sale for coming out of beta. At first, I thought MaruMori was too cute, too gimmicky, and the tone in the grammar lessons felt childish. I decided to stick to my 2023 resolve, and actually go through all the lessons one by one. Little by little, I found the grammar explanations stuck better because they weren’t written as bookishly as usual, and they give explanations of nuances which I had never read in my many grammar books. Even though I was wary of the Duolingo style grammar exercises at first, they amount in the end to translation exercises, instead of cloze, and I need both for my learning style for grammar, and always find that there aren’t enough for me in traditional books.
MaruMori is different from WaniKani and BunPro because it is set up as a complete curriculum with grammar, vocab, kanji, reading exercises (the only missing part is listening comprehension). I think some parts are weaker than others, for exemple I really like the vocab SRS here on BunPro because of the cloze sentences, and the kanji breakup mnemonics are better on WaniKani. If you are using a textbook, you probably don’t need MaruMori. But if you don’t (and I can motivate myself to study on the site in short, 10-minutes chunk throughout the day, but I don’t have the time or energy to open a textbook when I come home at night), I think it’s a great alternative.
TL;DR : BunPro and WaniKani are mainly SRS focusing on their own particular subject, and they are very good at that; MaruMori functions more as a complete curriculum, textbook-like. I find they complement nicely for my learning style, and the cutesy design grows on you (grew on me, at least)