i practiced yamanni ryu with toshihiro oshiro for almost 20 years in california, and chogi kishaba for three years in okinawa. i would have to agree with you that the difference in efficacy of yamanni moves with those of other okinawa-based bojutsu styles is "night and day."
at the heart of the difference is the flowing and rapid-fire characteristics of the yamanni swings; just as important is its emphasis on adjusting the footwork and length of the striking end of the bo to hit opponents extremely far away, as well as opponents who can be chest-to-chest. most other styles emphasise a 1/3-1/3-1/3 grip, which eliminates the bo's biggest advantage over other weapons: its length.
also, i think it would be apparent to others if they had some form of kumibo incorporated into their training that choppy, rigid moves are not effective against opponents who are accustomed to fighting with weapons (ie, swordsmen, other staff fighters, etc.)
i took up naginata after approximately a decade of training in yamanni ryu. i did this because i wanted to get a sense of what it was like to fight against someone skilled at fighting with a weapon. you often see bojutsu demos and pre-arranged fighting with bo, but you seldom see real "sparring," or kumibo. i thought naginata was a close as i could get...and it would be a good way to explore the efficacy of my bo techniques against ppl used to fighting with similar weapons. within a few years i was fighting and doing well in naginata at the natl as well as intl level.
yes, naginata and yamanni moves have many differences between them, but they have many more similarities. both make ample use of a long striking side of the bo; both emphasize adjusting length of the weapon with hands that slide up and down the shaft; both have flowing footwork; the similarities continue.