It sucks to be a clueless person in someone's guard. I tap judoka from guard all the time.
Otherwise it is pretty neutral position. If I control your hips there nothing you can do to attack. I tap less experienced people with Ezeqiel, Trusting choke and Kimura from their guards.
I think the reason is more to get sambists acquainted with closed guard rather than use attacks from guard.
I wasn't talking about clueless people at all. I'm certainly not one. I have to do much more work defending closed guard than open guard. It's like being outgripped in Judo (actually realizing this analogy improved my closed guard).
Maybe Uchi Mata can comment better, but I think being in a closed guard is a disadvantage even at higher belt levels. A few black belts in my area have guards as dangerous as as if they have your back or you mounted.