There are probably lots of weird shapes used in practice, but the nice thing about game design is that you get to build the mesh directly instead of making your best guess at what kind of surface unconnected points ought to represent. Mapping to a sphere is definitely pretty restrictive.
I'd imagine most are either planar or complex/undefined. Most meshes in gamedev probably aren't even closed with a unique inside/outside; just "pile of triangles".
If this aids your argument, nearly All video game models that I tried to convert to 3D printing STL files were non-manifold.
I think this logically follows from your statement, and it made fixing the stl files necessary otherwise the printer would switch to imaginary coordinates halfway thru the print and since the printer is mechanically restricted the print does a good impression of an Escher painting.
