Take the edge-split modifier for instance
In the viewport (upper left) this object (which has 4 materials) shows up perfectly with the edge-split modifier
However what you see in the rest of the screen shows what happens in-game, it gets messed up.
Download the .blend in the WIP forum and see for yourself when using the edge-split modifier. The only way otherwise to get the sharp edges is to apply the modifier, but I do know it’s easier to work when you can just have the modifier in the stack.
This is an old game and I think it may be a problem with old games, it also messed up like that when using the subsurf modifier and pretty much any other modifier. Is this a bug that needs to be fixed?