We carried up Rhyd Du and rode down Rangers at the start of the year.
I personally wouldn't even bother attempting Rhyd Du as a descent.....the penalty for failure along the (often unridable) ridge line is massive, a good deal would be a carry and I think it would be very disjointed. Saying that, sometimes things look a whole lot easier when the bike is facing downwards.....
Rangers is techy, but potentially cleanable and thoroughly enjoyable. The best natural descent that I have done in the UK and probably one of my favourite all time trails.