I fitted 20mm risers today - no problems with cables etc.
I managed to drop one of the large washers on the underside of the bar nut & it's vanished. I've ordered a new one but I'm more worried where the old one is & what it's going to do on it's way out........ So keep hold of all the bits when you're removing the nut on the underside.....
I fitted the up and macs Wunderlich risers, no problems with cable, you just have to change the attachment point of one cable, for which instructions are provided.