I think you make a good point but I'd seems unlikely that a bucket or two in the corner of a yard per year would be a scale that would be a problem.- .
So how do you responsibly discard the waste from a composting toilet? The answer is this cannot be a zero impact solution and at any scale begins to become a serious health issue.
many of us poo very seldom on the boat but proper handling is important and volume matters, agreed.
For many of us it seems to be a solution if Handled responsibly.