It is hard for developer to support elaborate setups (including Panning) in one instance using multiple cabinets to recreate a complete setup in one instance (eg. 2 cabs both with individual setting and panning).
This would be great for other uses as well, but for me, Cabinet is the main use I'm interested in.
Additionally, further support of Stereo use IMO could somehow be improved, I find that Stereo image almost always need to be re-processed after using Nebula on Stereo channels.
I am looking at developing chains as you describe and some amps may come in the future.
As for stereo processing that may be an error that creaps in if a developer does not adjust the subtle level differences between the left and right channel. It manifests as a stereo image shift. Very little can be done about it once a program is made as far as I know and it isn't nebula's fault per se. The options as far as I know are for the producer to redo the program after adjusting for the level differences or for example, you can use a panning plugin with Gonio to correct the shift issue.
I have this exact issue with one of my packages that will require re-doing.