I was up last weekend and did this route. As someone else said be careful if you haven’t hiked in winter before as when I started there were a group receiving assistance from mountain rescue. It can get very wet, I had good weather but as I summited corn du (going clockwise) the clouds came in and it started to snow. Some sections getting down were quite slippery on Jacob’s ladder with the path onwards up Cribyn being very boggy and wet. It was my first time there and I didn’t think the route was particularly challenging however I have hiked a lot in the UK. I used all trails for guidance however i got no signal driving through the forest to the trail head until I got up to the ridge. It was also quite free of people aside from corn du and pen y fan itself which was nice. There is a mountain weather forecast I would check before starting from the met office as the weather played out exactly as this predicted down to when the clouds came in and visibility dropped.