In an ideal world they are just someone's opinion with the qualification that the reviewer attempts to be as objective as possible. However, contemporary review sites like Kotaku and Polygon, and increasingly Gamespot and IGN, are ideologically driven. As far as those YouTube reviewers I mentioned go, they are trustworthy because they do not reduce their reviews down to numerical scores (while also avoiding ideological or overly subjective reviews), but instead spend 10-15 mins not only telling you about the game, but showing you some of it too. I find this a much better way of deciding if I will buy a game near launch or not.
The crazy thing is that when it is ~£20 the game will most likely provide a better experience after a few patches, hopefully anyway.
I also tend not to take any notice of negative reviews,each to their own and I think it looks very good game.
Ultimately they're just someone's opinion. Numerical scores aren't that important, just a guide. I've loved playing games with 60 or 70% and been bored to tears by games with 90%+.
I tend not to really care what review sites say, they are kinda useless, as the Days Gone reviews showed. There are a couple of YouTube reviewers who are trustworthy, and then there are gameplay videos to go on.