I am not an attorney and any comments I post are not intended, nor should they be construed, as legal advice. If you need legal advice, please consult a legal expert who is familiar with the area of legal expertise you need.
From a legal standpoint, the Federal Trade Commission (FTC) guidelines make it clear if you are reviewing a book you got as an alpha or beta reader, or an Advanced Reader Copy, then you need to make it clear in the review that you received it as such. The below is from my Disclosures/Disclaimers page: https://upsdownsfamilyhistory.wordpress.com/disclosures-and-disclaimers/
Federal Trade Commission (FTC – United States Federal Government Agency) Guidelines: Why Bloggers, Websites, and Others Who Use Affiliate Links, Offer Product Reviews, Etc. Should Be Aware of the FTC Guidelines
This applies to the author as much as it does to the people doing the reviews. However, the author will be the one dealing with fines imposed by the FTC.
On a similar note, Amazon and probably other book sites have rules about clearly marking your review if you received a copy in this manner. They won’t publish the review, but it’s important to realize the author could be penalized by Amazon for you not disclosing it. That’s on top of the FTC fines.
I have seen a few authors encourage people they give copies in this manner to not indicate in the review how they received the copy. That’s a major violation of the FTC guidelines. I don’t do reviews if I get a copy in any of these ways. It only causes problems for the authors and I find it distasteful for any author to suggest it. This applies even if you leave a bad or average review.
I gave up trusting reviews a long time ago. I think the least trustworthy reviews come from paid reviewers like newspapers and major websites. Too often they will give a bad book, movie, or TV series a much better rating than it deserves. After reading a review by a local newspaper reviewer about a movie I was seeing minutes after reading the review. I don’t know what movie he saw, but his review was out of sync with the movie he reviewed.