The complication is entirely in storing suitable header information - or at least enough to make a good codec selection decision.
A splitter is only as useful as how many stream codecs it supports. That's why the Haali changelog is full of things like "Added support for muxing FLAC audio as A_FLAC to the muxer" and "Added support for PCM audio with 7 and 8 channels".
It doesn't mean the guy has spent months implementing 7/8 channel PCM support. It means he has tweaked his splitter to support identifying those streams. By means of a some specific header information.
A splitter is directly responsible for offloading the decoding of a stream to the appropriate codec. Therefore, if it chooses incorrectly, you might find that DXVA doesn't work. That's why the best splitters (such as Haali) are popular because they are so damn good at identifying streams correctly and passing them to the correct codec. The authors, as you can see from the changelog, puts so much time into ensuring compatibility with even the most niché codecs.
It really is as simple as this.