See comment about about the important of sensitivity analysis.

It is really hard to come up with an absolute level of uncertainty. there are so many layered assumptions, each with their own uncertainties, in an assessment. What is important is whether a small variation in an assumption would lead  to a meaningful change in results / conclusions.

Uncertainty is also very context specific. So in my view this is also about application of coefficients. A coefficient can have inbuilt uncertainty, but that doesn't mean its application has the same level of uncertainty. It will almost certainly have more!