I've been stymied twice by nav near the summit of Pen y Fan. Once was about 6 years ago when my glasses went into terminal steam-up mode in very cold clag and someone had to help me down off the summit of Corn Du because I couldn't see. I immediately resolved to resolve my squeamishness about contact lenses. The second was earlier this year when we were at Bwlch Duwynt in a complete white-out, my partner didn't have her goggles with her (and they weren't much help anyway), so we just went back down.
"Instructor number two... has climbed Pen y Fan over 35 times now" not very many then 
so perhaps that's why the nav is regarded as 'notoriously tricky'!
I frequently meet people up there who get disorientated in the clag. Many have no navigational aids, or do, but haven't used them. There are clear ridge lines, lots of paths and easy escape routes. And I can recommend much better places in the Beacons for tricky nav!
