Bird pose is probably the most difficult posture in Flow. I don't do it as I have a wrist issue so, Bird is totally off my yoga pose list. I'm glad its not used that much as its really not that necessary to do with the general population as the majority of people cannot achieve it.
My hips are flexible so, the hip opening poses I don't have much trouble with. Cow Faced, Full Swan, Frog, Half Lotus...those feel wonderful to me. I can handle Splits {on both sides} but, I wish I was a bit more flexible to get a better Standing Split. I tend to do better with the floor split than standing.
The most difficult postures for me are the balance poses. I have eye problems so, for me to set a steady eye focus is almost impossible. I fight a lot with very simple balance poses. In tracks where the balance poses keep floating from one to another such as the balance track in release 34 and 43, those are very very difficult for me. I've gotten better over the years but, will not improve with balance poses like the average person would if they did Flow as much as I do.
Oh and there's nothing like Lunge pose to FRY the butt and legs.
Feel the beat under your feet!