In my case there seems to be a relationship between the smoothness of the panning and the type of headphone cup. Closed ones are more likely to have holes between the ends and the center, open ones have a smoother transition. I don't know if the relationship is due to the type of cup or because the open headphones I have listened to are Hifiman and this brand has achieved this type of panning.
In the case of mismatch in the volume of the transducers, when I have experienced it with a difference of 3 dB, the soundstage has collapsed towards the louder side, making it impossible to hear them without correcting through the balance control of the amplifier or the source.
Yep, if you've got bad channel matching then you can forget about proper imaging and soundstage. About your comment on closed headphones, I only have one pair of closed headphones and that's the NAD HP50, I found that ok with that headphone. It's just the HD600 that had the most profound unsmooth panning, being left/centre/right and not much inbetween. Headphones are in my sig, but I'll list the ones that I own/experience: K702/HD560s/HE4XX/NAD HP50/HD600, & oh yeah HD800 (sort of a mongrel S version - HD800 with HD800s pads, but a very nice refurb)(and I'm still working on the HD800 so I don't have a fixed opinion on that one yet, but it does have smooth panning though which is the point we're talking about).
EDIT: And you mention Hifiman or open back being related to smooth panning. My HE4XX is Hifiman and that's fine in that department, but that's the same for the K702/HP50/HD800 & HD560s, so smooth panning is not a Hifiman only trait, and it's not an open or closed back trait either (HP50). Good channel matching is the main factor in my experience, and just on top of that you have the odd headphone design that doesn't seem to do smooth panning, ie HD600 in my experience - (but it would be silly for me to say it's a bad headphone because it's a good sound at stock which is quite unusual for a headphone).