Workers are paid on a somewhat continuous basis, so if stakeholders are unhappy with their progress, they can unvote the proposal. It's very similar to paying someone on an hourly basis for work, then stopping future payments if you don't like the work they are doing. Naturally this requires either oversight of the work or else some trust in the people doing the work.
In the case of someone you don't trust at all because they are "new", you could have the funds go to an escrow entity, that would only pay out the funds based on delivery milestones. But it's often difficult to find anyone willing to work on such terms.
Payment would be in SBD, so the value of that payment would rely on the value of SBD itself. Another worker proposal I'd like to see proposed shortly after this system is in place would be a proposal to fix the peg for SBD as has been suggested in the past.
As far as the risk goes, the least risky method would be to redirect some SBD from the current reward pool to the worker proposal fund as a number of people have proposed. In that case, no additional SBD would be created relative to what is created today.