Well, since I was and am, why bring it up?
What Islands book? If I can distract you from edition warring for a second, you'll note that almost all of the other 5E hardbacks all have more reviewers then SCAG.
Well, that's a bit obvious since the SCAG has been out for two weeks and everything else 5e has been out for months. I was referring to the latest Pathfinder release
Inner Sea Races. Sorry had the wrong name. Came out about the same time, has the same style of material as SCAG and has 4 reviews currently. Sure, it's got 4.5 stars, but, come on, it's 4 reviews.
This isn't edition warring to compare two very similar books - note the I09 review that was brought up in this thread reviews the same two books at the same time.
If what you said was the tautology that sales rates tell us more about sales then reviews, I'm not sure why anyone would argue with that. It's a little ridiculous to lead that with "star systems are a bit borked" as if we should or anyone does expect them to be a measure of sales copies sold.
Review systems are borked when you're comparing reviews by ones of people to reviews by tens of people. You're the one who has brought up the fact that every Pathfinder book has a higher review. My response is, well, of course it does when you only have half a dozen reviews, it's pretty easy to get a high rating.
How many people preordered this? I'd say that for series work, a bad book can sell well and do a lot of damage down the line, when people think twice about buying the next books instead of preordering them.
Again, how many is "a lot". How many people are actually disappointed by this book? Judging by the star reviews, everyone should love the Pathfinder book and dislike the WotC one. Yet, I09 pans both of them. How much of the poor reviews is due to unreasonable expectations.
Now, if you want to criticize the SCAG because of poor cartography, or bad writing, or bad mechanics, fine. But, that's not, generally, what's being complained about. People are complaining because this book isn't like some other book. Well, it was never claimed that it WAS going to be like the FRCS. I mean, good grief, the FRCS weighs in at TWICE the page count (or close enough). How much of the expectations of criitics is because they never bothered to actually look at the physical product before they bought it? Even on pre-order, you knew this only had 159 pages. What do you expect for something that's not a lot longer than some splat?