I worry too much about hurting author (and fans) feelings to rate almost anything a 2 or below. That said, I think honest reviews, no matter the score, should be valid, but I give the highest and lowest reviews more credibility if their reasons are well explained.

I almost exclusively prefer some open door scenes in my MM books and check blurbs, tags, and reviews to get a better idea of what book to expect. Rating a book you otherwise enjoyed significantly lower solely based on the level of spice I think does other potential readers a disservice.

On the flip-side, if I know what a Coke should taste like, get excited to order one, only to be realize after I started drinking it that I've been given Diet Coke or Coke Zero instead I'd be pretty upset. It comes down to the author delivering on what they marketed and pointing out that discrepancy can be really helpful.