there seems to be two ways of doing uke nagashi, tenkan and irimi, which do people think is more effective/realistic?
Irimi - tenkan is not the best way to describe the possible variations of uke nagashi. Soto - uchi (outside - inside) would be better. There are many variations possible (usually defined by three levels shita-naka-ua), and there in lies the strength of uke nagashi, not so much in selecting one or two that are supposed te be more effective/realistic.
Uke nagashi is a much valued technique in other martial arts as well, from Shorinji Kempo to Wing Chun.