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I've recently cabled up the brakes on my gravel bike (rim brakes) with whichever outers it was came with my Ultegra shifters (I think BC-R680).
The inners were a bronzey/brown finish but didn't feel or look as if they had a polymer coating, but descriptions online suggest they do. Either way they feel great, the lightest and most powerful of any of my brakes, but I'm wary of polymer coatings, they seem to have a bad rep, but was that just for gumming up shift mechanisms?
I think so, if you think about it the mech has far less force going through it than the brakes so any build up may be detrimental.
Saying that I've had polymer coated Shimano shift cables on my last 2 MTB's and have never had an issue