For me, horizontal dropouts are like a heavy bike. I need to ride a heavy bike so mere mortals can keep up with me. I use horizontal dropouts because I'm too good of a mechanic and I need more challenge in my tire repairs.

I have sliders, ebb and horizontal dropouts on several IGH bikes. IMHO, horizontal dropouts rule. No added complexity on the frame (and nothing to fail), only slightly harder to do a wheel removal (once the chain length is set, it sets wheel position in the dropouts). OTOH, what do I know, I'm a Bear fan and I still patch tubes :)

BTW, all the newer Surly frames allow wheel removal without loosening the caliper. I don't have to move my fender, not sure why that always comes up.