Well, we've long nuked the shit out of this topic so I'll add my two pesos.
In my opinion, since you can't replicate every exact flight/sim as the same for each student, NSS will never be perfect. That inherent flaw alone makes it an "unfair" system. It's not a college program where everyone gets the same test, in the same room, with the same lighting, same temperature, same noises etc... I got over that fact in primary. However, one thing that the NSS system does do well is that it identifies trends. When a student attrites or goes through a service level review, they don't just look at the flights said student downed. They look at the whole picture. If SNA is consistently getting barely passing or below average scores for general knowledge in the brief, failing tests, and subsequently experiences consistent brain-aneurysm like symptoms while flying, then it's probably not going to go well for the SNA. Now, if a student comes prepared (or shows a solid relative effort) and is a below average stick, then his grades might suffer and he might pop for a low NSS. That just means to me, the SNA will have to put more effort into his abilities in the future than the average student. Everyone has their days, and more than 50% of SNAs going through the school now get shot and get handed a yellow sheet. Shit happens. It always cracked me up when I heard a student say "man, that (insert stud xxx) is a good pilot"....I usually say "STFU, we all have less than 150 hours in the air, we all suck in different ways"