The email stated, and I won't use quote marks, for fear of being sued. I won't bore you with the details, but it was basically that the parameter passing order in JSRs a RETs was reversed, which meant that we could not use Emulate with our company and client software standards. This was many many years ago, probably circa Logix5000 Revision 10 or thereabouts, possibly even earlier.
I just wish I had kept, even framed, the email response I got from A-B in USA when I discovered that RSLogi圎mulate does a few things 'differently' than a hardware controller does.