Currently Traktor has no stems display by itself, so currently the ONLY controller that stems make sense is the S8.
NI said they will develop a stem display within Traktor, so when that comes along you can use it with any mappable controller.
After watching the video of Steve mixing stems for 2 hours I have to say I really dig what he did and I think that was the maximum you could do with stems currently… I am not impressed. He did not much that I could not do with a 4 deck controller mixing edits of the songs.
Same as with remix decks: Nice toy but rare to see anyone use them meaningfully apart from a presentation or demo.
So I will surely wait a year to see if this takes off.