Cable housings for brakes are usually made of spiral wound metal with an inner plastic liner. Derailer housings have lengthwise steel threads with a liner. The spiral wound brake housings should not be used for indexed shifting derailer cables because they can compress and cause sloppy shifting.
So I've had a couple of road setups where the compressionless housings have had some of the lengthwise strands work their way through the ferrules. I've tried several ferrules that are supposedly made specifically for compressionsless derailer housing, but still had some wire creep. So I just finished a rebuild on one of them using brake cable housing on the derailer cables. I'm planning to use this bike on a long trip as an experiment to see if I can notice any effects.
If somebody has tried this and definitely seen very sloppy shifting, ..then wave me off this potential misadventure. I just took it out for a short ride and it's shifting great. Doesn't seem sloppy yet.