Ian, the color developer and first developer are critical for correct color reproduction and image structure. I doubt if those publishing the formulas tested each and every film for RMSG and interimage.
In fact, the films are tested against the developer, not the other way around. It is not possible for an incorrect developer formula will give proper results with all E6 films. Maybe, by optimization, one or two will work close to spec, but not all.
E6 films are not closely related to C41 films. The C41 films use DIR and DIAR couplers to get interimage and enhance sharpness and masking couplers for color correction. The C41 couplers are designed to work best with paper (not by viewing with the eye) and are designed to work best with CD4. The E6 dyes have a spectral cut to suit the human eye and work best with CD3. The E6 emulsions are designed to work best in high solvent developers for optimum imaging and are designed to be developed to completion, whereas C41 films are not.
There are many more differences in the sensitization levels of the emulsions, the silver quantity used, halide ratio, etc. It is a long list.
The bottom line is that the formulas are only close approximations for E6, and will give only close approximations to specifications when used with most E6 films. The problem is that unless you do numeric or side by side comparisons, you cannot be sure of your results, only that they are 'satisfactory to you'.
Only Kodak or Fuji-Hunt chemistry passes muster on all counts.