Interesting and I'm still not sure I fully understand how they come up with ratings...
Last year I ran Porcupine Rim, Fins & Things, and Poison Spider Mesa. IMO Porcupine Rim was the tougher of the trails mainly due to the "pucker factor" in spots running along the rim on uneven, off camber, sections.
After running it on the first day I was worried what days 2 & 3 would bring being that both trails were rated 4s. Well, I enjoyed the 4 rated trails much more as I don't recall there being much in the way of pucker factor areas. Maybe the fact that they are more "technical" with some of the obstacles grants them a higher rating?
Sometimes I think for me to really enjoy offroading I'm gonna need a vehicle with an exo cage that I have absolutely no worries about rolling, denting, scratching, crushing, etc.