Patrick,
Nice thread. Good week for you!
While I love the high flashed bunkers, I come to the same conclusion you do - flat sand and steep banks makes for the best playing and maintaining bunkers, even if, as Billy Crystal would say, the McKenzie style bunkers look "Mahvelous."
What I am working hard on now is making sure builders get from 3-5% pitch uphill in the bunker base to at least make them visible, rather than in a hollow. At the very tops, within the last five feet of the bunker, I find I can flare up to as much as 12% without wash. 17% seems to be the absolute max, at least here in DFW with our combo of sand and rain. It might be flatter as close as Houston! Even those would wash in a really big storm (we had 5" the other day...I didn't check any of my bunkers, but I bet they washed under those conditions)
Also, one thing those 50's guys did better than currently (in my experience) is to make sure no water uphill of the bunker drains into the bunker at all. If enough surface water enters the bunkers, even flat sand can wash. So, I am really reducing my backing mounds for bunkers to no more than a foot or so above the sand, and making very sure that there is at least a 6" lip on all areas.
I know many here will shake their heads at my answer, but that is how a gca must think, as the old saying is "form follows function" (not aesthetics at all costs) and "the devil is in the details." If you aren't worried about these kinds of things, then you are really just an amateur architect playing in the sand box.
And, as Rich hints, not only is it not nice to fool mother nature, its damn near impossible! Bunker liners and what not help, but its obviously so much better to design within what mother nature tells us the materials will do on slopes.