I admit, I did these for books for a while. The e-mails always said an honest review was what mattered, and I always tried to review the book for the intended audience, as opposed to just me (a lot of the books I was sent were way, way outside the genre list I provided by the review service, even though there was superficial overlap if you just read the back-of-book blurb).
Besides the expected issues with thin-skinned authors, there was also “star inflation” to deal with. My standard “if it’s your kind of thing it’s worth reading” wound up being four stars, with five reserved for work I was actually impressed by and three (I didn’t dare go lower) for books that didn’t achieve what I thought they set out to do. All of those three star reviews got a terse “we appreciate your honesty” message from the review service… and then no further books for a long time.
I stopped reviewing because a) I kept getting pushed to read books I wouldn’t enjoy and b) even though the reviews were supposed to be “honest”, there was too much pressure to assign those four or five stars. It does make me a bit sad, because those few five-star reviews were for books that were real discoveries for me, books I thought were amazing but would otherwise not have read.
Having said all that, I’ve also noticed a flipside: books which are indie often get slammed with “needs editing” even when the form is obviously experimental – or even quite professional, thank-you-very-much.
As usual, a full disclosure of bias in the interests of providing context goes a lot further than a sliding scale of stars.
In the meantime, for Amazon, I recommend reading the three-star reviews. That’s where you’ll find the reviews that weighed the pros and cons.