It's an old one, but a good one - all you can ever do is use review scores as guidelines and not treat them as gospel. Try a game out if possible - if you like it, you like it.
I see what you mean though with regards to the reviews being complete - I remember a lot of old magazines would put in their rhetoric that they 'always played games to completion', then they later added 'if it is possible', then it seemed to disappear completely - you just take it for granted that the game has been played enough for the reviewer to form a balanced opinion of it.
If every game were to be completed before it were reviewed, the writeups would arrive months later than the game - as a lot of people do base their purchases on review scores, the reviews have to be as up to date as possible. It's a bit of a vicious circle..