Mike,
you raise some good points here.
The main advantage of rooting powder is the fungicide, IMO
I was told there were a couple of reasons for failure beyond what you
have mentioned. Too much powder forms a cap which affects air and
moisture reaching the root zone, which then affects the roots abbility
to break though the talc cap. This is one of the reasons I have moved
over too gel.
I take you point about other factors being major players, humidity,
moisture etc, but they are out of control for most amateurs (how many
of us can accurately control temperature - many domestic electric
propogators have fixed thermostats - my Scotts heats the soil to 24°C,
which explained my poor performance)