Well… FBX does import ngons, the problem is that Blender does not supports ngons with holes (nor ngons using same edge more than once, which is how ngons with holes are represented in formats like FBX or OBJ).

OBJ importer does handle that situation, but the issue here is that doing the same thing with a format as complex as FBX is not really possible (would be a nightmare to correctly map newly generated geometry to old one for all the datalayers). It’s already really complex code in OBJ, btw.

Think the solution here would rather be to add an additional step to our (C-coded) mesh.validate() func, which would check for those blender-invalid specific cases and modify the geometry accordingly. Would also be much more efficient than doing this in py.

@Campbell Barton (campbellbarton) if you agree, I can try to make a patch?