I think 'Goodness of Fit' AND the 'Slope' of Regression Line are the factors to look at. If there is a uniform bias over the range of the instrument, the regression line will be horizontal and if G-of-F is low, the line has no meaning. However, I couldn't find any guidelines as to the actual value at which the G-of-F can be called 'low'.
Again, the slope of the line may well be zero and Goodness of Fit > 0.9, but what value of bias should be accepted?
Help!