Reviews are allowed to be subjective. I have read a LOT and I sometimes read a book and see that objectively it’s a good book, but I just don’t vibe with it… so it gets 3 stars and if I feel the need I clarify why in my review. Just because a book is well written or has an interesting premise, etc. doesn’t mean you have to subjectively like it. Noting in your review that your enjoyment was limited due to struggling with the language, for example, is a valuable info for other readers that might be on a similar level to yours.
(5/5 are basically perfect books for me and are rather rare but everyone treats star ratings a bit differently. )
Re gradings: I try to only grade things that I recently read, because I remember how difficult (or easy) it was and if the book I am grading against was similar or not. I don’t actively consider grammar or kanji because if I struggle with them, it just adds to the difficulty level. If you notice things like these, it’s a good idea to put it in the review. I don’t really notice most of the time. Where, I think, it does make a difference is with or without furigana. A book that is Level 28 with furigana might be 30 or more without, but it’s usually not such a huge gap that it makes gradings useless. 
this might be interesting to you:
