From what I now know, there are a range of implant style experiences. I was having both upper rearmost gnashers renovated, both sides needing sinus lift where the bone in the sinus cavity is augmented with Polyfilla or something like that, the sinus lift elevating the process to difficult in all events.
One side was as bad an experience as I have ever sat through, involving 90 minutes of work; it is that side which is responsible for the chipmunk-pouch look. The other side was much quicker and less stressful, with minimal swelling which has now mostly subsided.