I do think the willow feeder system is close to foolproof.
The main way to mess it up is to get urine in with the sawdust and poop, but the problem then is smell, mostly.
You need to figure out a way to get a toilet seat up high enough (my favorite is when there are steps up into the willow feeder and the whole floor is whatever inches off the ground) and you need to figure out how to establish the right airflow if you want it to be smell-free. Those are the main issues.
Please consider pledging to the current Kickstarter so we can create a movie that explains all this! (link above)