I don't think we disagree too much. If I had to pick one, I'd agree with you that the rest of the uncertainty is likely larger in most cases, but I think you substantially underestimate how inaccurate these numerical methods can be. Many commercial computational fluid dynamics codes use quite bad numerical methods along with large grid cells and time steps, so it seems possible to me that those errors can exceed the uncertainties in the other parameters. I can think of one case in particular in my own work where the numerical errors likely exceed the other uncertainties.