How well are the scales determined?
Note that determination of scaling parameters depends on symmetry-related observations having different scales. If all observations of a reflection have the same value of the scale component, then there is no information about that component and it remain as a systematic error in the merged data (this may well be the case for absorption for instance)
Thus to get intensities with the lowest absolute error, the symmetry-related observations should be measured in as different way as possible (eg rotation about multiple axes). This will increase Rmerge, but improve the estimate of <I>.
Conversely, to measure the most accurate differences for phasing (anomalous or dispersive), observations should be measured in as similar way as possible