How is the parts availability on these, better/worse than Shimano currently ?
The gearbox itself, I haven't had to replace anything.
The shifter, either Zerode or there's a few European options. I've purchased a spare before, and that was like buying anything from os, no problem.
The cranks, same story as above.
Cogs and belt are still the original ones on the Zerode (about 2,400 ish km) and hardtail (about 800 km), but I have bought spares from Zerode. There's also the European stores.
Anytime I've had to buy something, which is really only spares as a backup so far, I haven't had any issues getting stock.