I've only had issues with one client, who is a national non-profit with procurement, etc.

The risk for the client is that you'll flake out. If you have a track record of being reliable (When I'm subbing out work, reliability and professionalism > technical capacity in most cases) then there's really no reason you shouldn't be getting prepaid work.

You're the one at risk. You risk losing the time spent working, which is non-renewable.