Hi Nicholas VH, I cycled the
Camino Frances from Leon to Santiago from 14 - 18 Sept 2017 on a MTB. There are definitely steep, rocky and technical MTB ascents and descents. On the
Camino Frances, the upside is there always is a road option given the volume of traffic the route receives.
The climb to the Iron Cross can be done on tarmac or off-road. Please take the road for the climb to O Cebreiro. This climb is split into 2 sections; before and after Laguna. Whilst the climb after Laguna to the summit on the walking trail can be attempted by cyclists, I chose not to as it is my legs have taken a beating and the 2nd part of the climb is not easy. I found the views to be better on the tarmac route for the climb to the summit as the trail has to be done through the trees.
The descent from O Cebreiro should be done on road till after Fonfria. I was able to use the walking trail to descend after that. The route leading to and after Arzua includes sections on narrow, steep, rocky paths. It's not just a matter of one's strength and endurance but also bearing in mind that the cyclist is amongst a multitude of walkers, all of whom are struggling with the uphill climb.
From Triacestello, the route through Samos is paved, longer but easier and you visit the monastery. The alternative northern route is shorter but almost 90% off-road with ascents and descents on gravel. I took this route and I don't recall a road option. It is the more scenic route though.
Before the bridge to Portomarin, there are 3 options - Principal (Historical), alternative and supplementary. I can only assume the latter 2 are rideable as the Principal route is not for cyclists - it includes an extremely narrow, steep, rocky ascent - I was thankful there were no walkers to witness my toil of carrying my bike and panniers down.
I was told that the Via de la Plata (the Silver route) is a more manageable route as it has less ascents and descents and off-road trails. It may even be done on a hybrid. In amy case, this route can be merged with the
Camino Frances from Astorga.
Hope this helps with your planning!