Oh sure there's always going to be some personal bias. But a good reviewer in any medium will seek to temper it and aim to bring their general appreciation of the medium into play.
It's just particularly obvious when you get a situation with a couple of extremely against the grain reviews as the very fact they're so out of alignment renders them suspicious. Of course equally you need to be wary of the opposite: glowing reviews for big franchises that seem to be given as a matter of course vs a smaller number of less hype infested critical reviews.
On the whole though I think with respect to The Witness it's pretty obvious it's a pretty fantastic puzzle game that is very well made technically hence reviews should be good to great. Anything less is likely more than a taste based "I didn't like it". For that kind of review you're better simply looking at average spread of user reviews instead.
If a critic doesn't have much fun with a game, they should make that clear and explain their reasons. If the game is generally critically acclaimed or not shouldn't have any impact on their opinion and, more importantly, doesn't make it any less valid. As long as the review/critique is well explained it absolutely does not matter.
I know many, many people who would absolutely hate The Witness and consider it a bad game, because it misses pretty much everything they are looking for in video games. If everyone would be praising the game, they and a huge numbers of gamers would simply be left out of the discussion.