I think Jaeger makes a couple of very goods points here about both spoil and also drainage. There is also the water table issue to consider.
There are many fairway bunkers, irrespective of lip height, on free draining links (and even heathland courses) Muirfield being one links course that comes to mind, where the ground for several yards around the bunker is sculptured to feed balls into the bunker, the base of which is well below the surface of the surrounding terrain.........but if you tried sunken fairway bunkers on say a poorly draining parkland or meadowland course I imagine you'd frequently get lots of water in the bunkers, hence presumably the trend particularly on modern-ish era inland courses for bunkers to be built within a raised-up mound. Indeed I recall seeing TV footage of the after effects of flooding on a flat inland course and it was very noticeable how just about the whole course, even many of the greens, was covered in floodwater but lots of the bunkers and surrounding mounds were poking out well up above the floodwaters, indeed the sand in the bunkers still easily visible.
Now my preference, topography, soil type, drainage etc permitting, is for fairway sand bunkers to be high lipped deep pits, genuine hazards. However, I'm also very much in favour of grass bottomed fairway bunkers, even if they're sunken ones without much lip, those at Huntercombe and on the Green course at Frilford Heath being examples. If they're dry they make shots a bit more challenging for the better player whilst the lessor player can play out of them okay, and if they're very wet, well you get a free drop from casual water in a grass hollow, but from a water filled sand bunker you don't, unless a local rule takes it out of play. With grass bunkers there's also less daily raking needed and less need for occasional reconstruction.
I hope my wording does justice to what I'm attempting to say! I shall try to find and post some photos by way of further clarification.
There is one more aspect that I would like to highlight however, and one I really don't particularly care for, and bunkers within mounds on modern-ish era inland/parkland style courses seem to be the main culprit. This is where a narrow collar of rough is left at the front and side edges of a sand bunker which then holds back a ball from otherwise rolling into the bunker. I hate this. I'd much rather the grass were cut right up to the bunker edge so allowing the damn ball roll into the bunker if it has the momentum to do so.
Interesting post Pat, thanks for raising it.
All the best.
PS - there's also the light and dark shadow effect of bunkers. Rans recent interview/photography class with Aiden has reminded me that from my eyes bunkers in darkness or in shadow always seem to look mean and evil, while those in bright light look quite a bit less severe.