We rode it in the ice a few years ago. As @reluctantjumper says, top section through the gap was sheet ice. We ended up ‘bottom surfing’ down holding onto the bikes. It was a bit hairy! Also the climb up to the moor and the descent down to the railway line were extremely icy in places. And very VERY cold water if not.
The 5km or so back on the canal through more freezing puddles to Tal-y-bont was pretty miserable!
Having said that, on a bluebird day with no wind, it’s a stunning ride. Rode it 27/12 last year in 40mph ish winds and sideways rain. That was, er, a bit epic!
I’d not be rushing back to do it if it was icy.