THIS. My clean-legged TB gets really bad mud fever on ONE foot (the one with the white sock). I am continually on his case about it, so I don't ever let it get beyond a cm or so of scabbiness at a time before I strip it. However with feathers.... I cringe to think about keeping that clear. I always heard it was horses with white socks/stockings that got mud fever more readily, due to the more sensitive pink skin, but only have anecdotal evidence for this.
Worst case I ever saw was on a black horse with several areas at least the size of my thumb print. I've only owned two horses that ever had any (my current older mare being one), but I've dealt with it on other's horses and it can affect any horse no matter the feathering or color. Worst case I ever saw was on a solid black horse. The only thing I've heard that I've seen evidence of is that feathering does hide the early stages of it forcing you to feel for it.