Not every review needs to include constructive critic! After all, there is only one author, but many readers.... Which means a review is to 99.9% for other (potential) readers... Basically to help them decide, if the novel is worth investing their time in.
If it was all about 'constructive critic', the author can simply delete the review after reading it. After all he has already read your critic. It wouldn't have any value left for him/her.
So a good review is something inbetween. The (potential) readers should find enough informations about the novel and your reasons for liking/disliking parts of it, while the author should be able to get some value out of reading your review for the same reasons.
Reviews exist to help other people to decide, if they should consume your product. Get it?
Why is everybody fine with spammy fake 5 star reviews, but feels offended by 2 star reviews that actually say something!? Remember, 2 of 5 is the same as 4 of 10... (simple math...)... 4 of 10 is 'slightly worse than average'... (let's forget that you basically start with 2 of 10 as condolence stars for effort)
This forum is full of review swap threads... Some do serious reviews, but in reality it's just "you give me a positive fake reviews and I'll give you one". I've seen one side give a 4 star review that was looking rather professional, and the other side gave 4 stars with a few neutral 'encouraging words' back with 0 chapters read.... etc..
Reviews are for reader! Not for authors! It's called REVIEWS and not feedback.
Authors somehow think the world revolves around them. lol
I'd never trust a review better than 4.5 stars. Bad reviews are way better to help me to decide if I want to read something, than your average "NICE NOVEL!, EXP, pls release more chapters, pls release more chapters, the best since sliced bread"-fake account spam.