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?
You can use DXL for Source Control right now if you don't want to actually edit anything out side of Domino Designer. Do this by exporting and importing elements as raw NoteFormat.
For DXL in a format that you can mess about with outside of DD you can trip up in quite a few places. I've not done anything too aggressive with DXL, so others might be able to give you better specifics. Quite a few are discussed here
The openntf project Design Catalog can be used for version control. www.openntf.org/projects/pmt.nsf/ProjectLookup/DesignCatalog
At lotusphere the lotus911 people mentioned they used the Design Catalog in combination with Trigger Happy. www.openntf.org/projects/pmt.nsf/ProjectLookup/Trigger%20Happy
I know there was an issue with Web Service script code not being exported in DXL. I'm sure that's only one of many. IBM has stated they have a goal of "someday" making DXL design exports 100% "round-trippable", but even with 8.5 are not there yet. There is a good discussion here, with postings by developers of design elements which do not export in full fidelity.