The main motivation is the lack of reflex mirror and the electronic shutter (i.e. avoiding vibration). When he first got the Sony, he ran into some limitations of Sony's tethered software, but new versions seem to have fixed that and he can now capture tethered images and have them deposited into a folder on his computer where his stacking software can find them (rather than having to move SD cards around).
At some point the Sony people were telling him that live view and tethered capture were not going to be supported on his camera and that he would only get that if he had purchased the next model (presumably the a7r III or IV).
The Sony A7R II is a full frame camera without an anti-aliasing filter on the sensor. This means it has a 42.4 megapixel sensor. Joe says it has significantly better low light sensitivity than his rebel did and seems to have no issues with noise. The body only sells at BH photo for $1600 as of March, 2021. Compare this to the A7R IV with 61 megapixels that sells for $3000.
Tom's Mineralogy Info / tom@mmto.org