The system argument is the go-to argument against picking up an A7 body when your already bought into a system. With the A7r II, there are more and more reviews stating that it performs as well or better than comparable Canon bodies when focusing with adapted Canon lenses. I think that invalidates the system argument...in the case of the A7r II. It may be that future Sony Alpha bodies get the same AF improvements, and if they do, then I think that would weaken the system argument even more...but for now, I am only referring to the A7r II.
Lenses are the primary thing keeping most photographers in their chosen system. There are flash and other devices, sure. Most of my non-lens devices that I have for my Canon kit can be easily replaced...if they need to be. For example, I have some intervolometers that I simply wouldn't need to replace since that's build in with the Sony cameras. The single largest basis of value that I have in my Canon system is the lenses, by a long shot. For some people who shoot portraiture and the like, flash might be a big second, and if it is, I can see a big reason for sticking with the brand (i.e. E-TTL).
Regardless of that, however, most people's investment is in lenses. Lenses are the things that stick around when we switch in-brand bodies even.
So, if your able to use your existing lens kit, and are able to focus as fast or faster with an A7r II, then the "AF speed, focus accuracy, and distortion/sharpness across the frame" are not going to differ. It's the same lens, so distortion and sharpness are certainly not going to differ. AF speed is going to be similar if not better. Focus accuracy, well that is as much the user understanding how to use the AF system as it is the AF system itself (and that is true for using Canon lenses on Canon bodies even...my 600mm f/4 lens on a Canon body is designed to focus inward before it focuses outward...before I knew that, I missed a lot of BIF shots because the darn lens would focus the wrong way when the bird was dead center in the lens, just OOF. I had to pre-focus the lens out towards infinity with my thumb before autofocusing, and I was nailing BIF on a regular basis.)
Is the A7r II perfect? Hell no. Will the A7r III be perfect? Hell no. But my 5D III is far from perfect as well. The point is simply that I can use the A7r II for what I need it for...mostly manually focused with a few AF landscapes, macros, flora and other nature stuff, with all of my existing lenses...no brainer. For the other kind of photography I like, still life scenes of classic old equipment and such at low ISO, where I do use AF, the A7r II will perform better with my existing equipment than the A7r (which, while it could AF, did so poorly enough that I ended up resorting to manual focus for all of it.)