I use them on my Optima Orca, which has quite a high BB. I find them reasonably secure, and rarely lose a pedal even on rough off-road surfaces. While SPD pedals are more relaxing once clipped in, the flatties are much more secure when starting off.
I wouldn't use them on a trike, where a dropped leg might cause serious injury, and I take Kim's point about extra effort/concentration which might become an issue on long days, but on balance I find they work for me on my habitual short rides.