Honestly, the worst thing about the CT6, as Don notes, is the size relative to around town driving, parking, etc. It is also one of my least favorite things about my Model S. Admittedly, it is great for long distance cruising. A quick comparison on dimensions:
2016 Volt / Model S / CT6
106.1 / 116.5 / 122.4 - Wheelbase
180.4 / 196 / 204.1 - Length
71.2 / 77.3 / 74 - Width
My Tesla fits comfortably in my garage lengthwise, but I would not want it more than another 4 or 5 inches longer or I am forced to move work benches around, etc.
I also agree with Don with regard to charging IF you have to do it a lot (the conundrum there is that "frequent use" is when you benefit most from it being free...at least for cars produced up until now). I have only supercharged about 3 times now. In two of those cases we ate lunch while charging so it was absolutely painless. The last time, however, I got to sit there for 20 minutes while I added another 72 miles. Hardly a deal-breaker and some would say "that's only 15 minutes longer than a gas stop") and that is true, BUT in the case of a PHEV I would likely not have had to stop that same day during the middle of the trip as it was 110 miles there and 110 miles back. Sure, I have a P85+, but at 70-80 mph speeds we would have run out of juice about 22 miles from home (note: I had charged to "255" before leaving home). I could have supercharged for about 10 or 12 minutes instead, but who the heck wants the added stress of coming home with range in the low teens or single digits. None of this is a big deal for a one SC stop situation, BUT if you have to chain them together for a longer trip I can see where it would be a lot less exciting.
The flip side for the CT6 (for me) would come down to daily driving. I would ideally want to be able to do 100% of my daily driving on electric regardless of weather. My first gen Volt just covered my commute, but often bled over to gas with any errand, going out to eat, etc. The CT6 would have the same issue for me (only worse) in the same use case (note: my use case is different now as I drive a lot less). We have all said that the gen 1 Volt is a gateway drug to wanting more electric and even GM has said as much with regard to the second gen Volt (and Bolt)...hard to accept less than a gen 1 Volt these days.