Nothing wrong with a horse sweating well during appropriate exertion and/or heat! Downside is that this sweat does not exactly benefit the durability of a leather or synthetic harness, because of the (light) acidity and minerals present. Fortunately, there is a practical solution to this by using underpads. So these offer even more benefits than comfort alone, which is why they come in virtually all harness components and in a variety of materials and colors: https://www.demenner.nl/?s=onderlegger&post_type=product&dgwt_wcas=1&lang=nl
So the idea that only a leather harness needs to be protected from the action of salts from sweat is not complete. Synthetic harnesses will also last longer if protected by a breast pad and/or an saddle pad.
