I'm guessing that you guys didn't have a squad just pack up and say 'Let's kill that Sunderer', and lay down fire to suppress/kill the engineers while bombarding it? You didn't specify any specific tactic you had, so the only one I can go off of is the strategy that I see the NC/VS use all the time of 'Run up one or two at a time and hit it with a rocket then run out of ammunition because they've got engineers repairing it', and that's exactly the wrong thing to do with any kind of active, defended Sunderer.
And my point with MRAP vs Tank was the MRAP is designed to resist multiple kinds of explosives, including RPGs, and keep going. Sure, an MRAP would be gutted by an M829 or M830 round from an Abrams, but the same can be said for the Sunderer. Tank vs Sunderer, the tank wins, as it should. But for infantry trying to kill it, yes, you actually have to work with other people. A Sunderer by itself is not invincible, but one that has a lot of infantry protecting it and repairing it? Yeah, that's pretty hard to take down, because it should be. They are focusing their efforts to keeping it alive, so why shouldn't you be expected to need multiple people to take it down yourself?
Sordid, it may in fact be a bug or unintentional that top armor is 'stronger' than the rear, or it may just be a side effect of how it's coded. Still, try the rear, it works wonders. And for C4 being a shaped charge or not, it could very well be that it is in fact. I assume it's not because of how often I notice it just makes a 'bloom' explosion that does equal damage in all directions. Of course, it could be some future successor to C4 that is able to switch based on situation between multiple detonation styles.
Edit: Typographical errors, why do you haunt me.