Whenever source control in Lotus Notes development is discussed, it is said that an export-import cycle of DXL data doesn't give you the same design as you started with - thus any system relying on DXL will fail. 

I have no reason to doubt this, since the DXL format seems to be a moving target and constantly a step behind what the Domino Designer can express, but: 

Does anybody have a specific example of what DXL cannot express?