Concave polys

Does anyone know what Unreal engine's position on concave polys is?
I mean polys on a brush as it's built, not the polys that result from subtractions which are AFAIK several BSP polys that the engine edits in one go as a convenience to the user.

I thought they were a big no-no, but I was fiddling with a new build of the Extruder (no surprise there!) and a concave shape I made looked fine.
I've made a map with some examples.
Some display bad polys, some seem totally fine, but I'm not sure I've looked at them under all possible angles

Look at the green cylinder from where I've put the light -- above the missing slice, looking down at it. It's hard to catch it right, but the engine draws the polys across the gap (across the mouth if you think of it as pacman )

one of the subtracts next to the main box is bad too.

I'd always thought a brush's wireframe had to be made of convex polys, it's just when I was fiddling with the extruder I made one of the shapes in that map, and I can't find the right spot to see the BSP error.
Since I already knew that vertex-editing a cylinder into Pacman fouls things up, it got me thinking maybe some concave wireframe polys were ok and some weren't.

Written a neat function for the extruder that tests to see if the shape is convex.
I was going to use that to write something that will divide up a concave shape into convex polys. Polys rather than triangles, so the wireframe isn't as messy.
This would then divide up the cap polys for the extruded brush so a concave shape could be used.

is the dark bit a sort of gutter thats lower than both of the things around it? Hard to tell on that pic.
the 2D editor will always split a brush up into 'nice' polys that the engine can handle.
if it is a dipped area, that's a lighting engine quirk -- narrow trenches don't get terribly well lit. select those polys and in the flags, try HighShadowdetail or bright corners.