The South North route is tough, and it is always TOUGHER than I remember, but IMO it is significantly more than 50% ridable, maybe not 80% though. Having said that, we have only ever cut around the lower slopes of Beinn Mheadahoin when descending from MacDhui, from about the Hutchison Hut, aiming to join the Lairig path as high up as possible. That’s not particularly easy going either… Fords of Avon must have been tasty this w/e with the melt going full.
And the East bank of the Feshie is classic trail. Did you do the loop in a one-er? If so, well done.
EDIT: From what I remember, the key to riding the Lairig path in a northerly direction, even on the equally frustrating ‘flatter’ stuff, is that you need to somehow keep up some kind of speed/momentum whilst avoiding tooth-shattering crank-strike. Not easy. IIRC the rocks are mainly securely fixed in the ground, and very awkwardly spaced. 29er?