DSX Wides doesn't work on all material because the impression of a larger space doesn't work with things like outdoor scenes in a movie. A good example of this is the opening few seconds of the movie 'Cliffhanger', where it sounds exactly like a helicopter being powered up in a large hanger. Problem is, the opening shot has that helicopter above a mountain range. The effect of being indoors is uncanny (Audyssey really did their homework), but doesn't work for this scene.
That problem is basically non-existant with Neo:X because it is not attempting to do room simulation but instead bridge the gap between your Front and Side speakers, like a Centre channel bridges the gap between your Front L&R speakers. When there is common info in adjacent channels, then that info is sent to the Wide speakers. So if a sound was to pan around the room, it would go from the Front to the Wide to the Side to the Rear and so on. When there isn't any common info in the Fronts and Sides, then the Wides are silent. As such, they isn't material for which they don't work, since they are only fed sounds that would have phantom imaged there anyway.
If you have a receiver that has both processing modes, there's no reason why you have to choose one over the other, when you might prefer each depending on the source material. Just understand why one of them might not be appropriate for certain material.