Actual numbers are never easy to come up with in situations like these, but some of the uncertainty is in whether or not priors of zero or one are bad, and some of it's in the logical consequences of Bayes' Theorem with priors of zero or one. The first component doesn't seem especially different from other kinds of moral uncertainty, and the second component doesn't seem especially different from other kinds of uncertainty about intuitively obvious mathematical facts, like that described in How to Convince Me That 2 + 2 = 3.