Does the idea of S-Ranking missions in Peace Walker seem more reasonable once everything is upgraded? It seems pretty intimidating at the point I'm at (just beat the tank).
If we're talking about main story missions, those will be an absolute cake walk once your gear is good enough. You're going to be able to put down the tanks with like 2-3 rocket shots believe it or not!
Some of those Extra Ops tank/mech battles are extremely difficult to S-Rank though and I remember people talking about how some were pretty much impossible unless you played co-op.