I actually have a bit of trouble with butterfly because I have pre-existing lateral meniscus problems on both sides. I can't let my knees get fully flexed or bad things happen. I still play butterfly as my primary open guard, however, because associated twisting in DLR/RDLR is worse and spider is hell on my fingers. Generally, I'm fine if I can keep enough muscle tension to maintain minimum distance between my hips and my heels.