lautaro - There are a number of submissions available from within the guard, but unless you have a significant advantage over your opponent (size,strenth,skill) I wouldn't call them high percentage.
Passing the guard isn't an easy skill either, for that matter, but once you've passed the guard you can get a strong position from which you can more safely attack your opponent.
Lautaro
I agree that the size of your opponent is a big factor in dealing with submissions from within the guard. Im a pretty big guy (6'2) so if someone smaller tries for example, the ezekiel from the guard im usually able to just push him away with my legs/hips. But a guy who is just as tall or taller can sometimes hit me with it just because I am unable to push him out far enough due to his longer arms.
Rhymenoceros - Actually, I have a question about this.
Back in the day, before the OG existed, a lot of us posted on the MAWN forum on AOL. Mark Tripp, a judo coach and cousin of Ron Tripp (the guy who beat Rickson at Sambo), posted a technique he said was a guard killer. I tried it at the time with some friends, but haven't really thought of it since.
It's called a jaw lock, and it works like this: start in the "stalling position," ie, head down, hands in uke's armpits, and work your hands around behind uke's neck like you're going to do a can opener. Scoot the crown of your head up under the jaw of uke, and then with your hands clasped behind his neck, pull down and "look up" at the same time. The effect is that uke's jaw feels like it's going to break.
Any thoughts on this? I swear to god I'm not making this up. There was even a judo guy who claimed to beat a BJJ guy or two using this technique.
I've used the jaw lock a lot in the past, and it's sheer hell. It's kinda like a heel hook: If someone doesn't mind breaking their own damn teeth and jaw, let alone getting their neck cranked off they could sweep you before you get a finish.
Watch the end of F Shamrock x Lober 2, Lober taps to a Jawlock.
But if you got a good base, you'll tap most people, but it's not a lot of fun to put on people in practice, so I stopped using it.