Well, the chromatic aberration means that the focal length changes a bit across the wavelength range.
The focal length of the objective is given by the focal length of the tube lens divided by the magnification of the objective (of course!), and a change in focal length is basically equivalent to moving the object by the same amount.
But because of the enormous longitudinal magnification, the effect on the image is correspondingly greater. In other words, any chromatic aberration in the objective is ENORMOUSLY magnified.
For a 1:1 relay like the Optosplit, this effect just doesn’t happen, and for the lenses we use the chromatic aberration should be down at the diffraction limit anyway.
However, once you split an image between two wavelength ranges, the objective’s chromatic aberration hits you right in the face.
Especially true as you go out towards the red, where cameras see images far better than we do, and the objective’s chromatic performance tends to be getting worse in any case!
So, our corrector lenses act like “spectacle lenses,” to compensate for a problem elsewhere. They therefore need to be selected by straightforward trial and error!
And as for flatness of field, EXACTLY the same argument applies. A tiny error in the objective across the field is enormously magnified, and in practice will swamp any such errors in the relay.
So finally, this isn’t a specific problem with anyone's objectives! Everyone’s do it to some extent.
Customers who do accurate spectral scans have to use something like a PIFOC focuser to move the objective during the scan to compensate for all this!