I haven't read any review, so no idea what is exactly the case, but a single flaw can easily lead to a big drop in score. The amount of flaws doesn't matter: 1 single flaw can already render the game unplayable for many, and then he's not going to say "This game deserves an 8! It's completely unplayable, but hey, unplayability is the game's one and only flaw, so what does that matter?". No, the game would get a 3 at max instead.
I see that the problem here is "lack of checkpoints"? Well, that can be a tremendous annoyance (for reasons other than abusing resetting). It wouldn't render a game unplayable, but to those who can't stand that, it does prevent the game from being great. A score of 6.5 perfectly resembles that.