Or is it?
I remember being mildly amused that the excellent book Wild Swans, a personal history of three generations of Chinese women in the wider context of the Cultural Revolution, got almost entirely 5 or 1 star ratings. People who hated it considered it to be a tale of falsehood for political reasons.
This is less likely to happen with fantasy, but because there are sometimes religious or political allegories (my personal favourite being Narnia, where Jesus stars as a talking magic lion) this could occur.
I never review a book here if I don’t finish it. The reason is simply that a book can start brilliantly and nosedive, or start shakily but become fantastic (The Lies of Locke Lamora is excellent but the start is a little slow). Sometimes reviewers don’t actually review the whole book, but only the start.
There’s also the occasional person who is clearly mad as a bicycle, and finds something incredibly strange or insignificant to get cross about. Happily, this doesn’t happen often.
A more fantasy-specific problem is that there are a huge number of subgenres. If someone buys a book expecting Abercrombie-style grim realism and find it’s more sword and sorcery they might give it a lot rating, even if it’s an excellent sword and sorcery yarn. That’s more a case of the buyer not doing a little research beforehand (maybe downloading a sample if possible), expecting butter, getting cheese and complaining because it’s impossible to spread.
The real problem is one of taste. No two people have quite the same view. I know a chap who shares my enjoyment of Mr. Abercrombie’s books, but I was staggered when he revealed he thought Mr. Martin’s A Song of Ice and Fire was tedious.
I think there’s also a trend bias. Right now plucky farmhands are out, gritty woe is in. So, this means it’s probably easier to get a better set of reviews of you write like Mr. Abercrombie and less so if you have a more old-fashioned black and white view of morality. This would also reflect the approach that agents/publishers often take when selecting and rejecting potential books and authors.
The ability to download samples (or read them online) is a real boon, as a prospective reader can get a feel for an author or book without forking out only to discover a dud.
I think reviews work pretty well as a guide, but they should be taken for what they are: a collection of subjective opinions which may or may not tally with your own view. In fantasy, where there’s huge variance in terms of magic prevalence, grittiness, modern or archaic politics and dialogue and so on it’s well worth reading a few reviews and finding out whether people believe the writer is a bit feeble, or whether they dislike the book because they wanted dragons and witches and ended up with political intrigue.