My practice seems to be stuck in an endless tug-of-war between backbends and leg-behind-head. If one is comfortable then the other is not. Today's verdict: backbends comfortable and leg-behind-head definitely not.
I suspect there are good reasons for the tug back and forth. The nerve cleansing aspects of intermediate probably have a lot to do with the motion of the spine...deeply bend back...then deeply bend forward.
I asked my teacher if this tugging back and forth was going to end at some point. Would there come a time when both backbends and leg-behind-head were comfortable in the same practice? He said "Yes"...and then followed up with "and I'll let you know when I get there." Hmm...seems like I might be here for a while then.