One reason carb heat cables and mixture cables tend to break is because they are set up with bounce in them. That is to say that when they are in the fully cold or full rich position the knob in the cockpit is not flush against the panel. This is to ensure that you reach the full stop position before the cable reaches its full travel, if you see what I mean. Now pilots seeing this think that it should be flush and may be think it keeps backing out, so what do they do, as they have never been told this before, they keep pushing it back in against the panel. This causes the piano wire to keep bending against the full stop position and if you keep bending a wire it breaks.
Just my two-penny worth.