I have often wondered that too (will it work in a real situation). During training, I have had some ukes fall rather easily for me when doing techniques, others didn't fall like I would have liked them too. As time goes on, I have disovered that uke (or even a real attacker) has to be committed the whole way through. I have noticed the more committed the attacker is in trying to punch, stab or kick, the more effective the techniques can be. Just my 2 bits worth.
Sincerely,
Eric Joyce
Otake Han Doshin Ryu Jujutsu