When I first got a Hope Vision 1 (which I still have, in fact), I experimented with using it in flash mode. I found the mere reflection of the flash on an overcast day disorienting. In fact, possibly worse than looking at it from the front. So I didn't use it in that mode again.
As for flashing rear, I'll use one when I have to ride on a fast, busy road. It's never occurred to me to use it in an urban area; and there are enough cyclists riding around town with no lights at all, or with just a rear light or just front, or a red light at the front, or various other unorthodox combos, and somehow everyone sees them.