Imaging Resource has done some tests to measure the image quality that comes from “sensor fleck” affected Canon EOS Rebel T6s & Rebel T6i cameras that was first reported by LensRentals.com. A brief follow-up from LensRentals.com states that Canon is taking the issue very seriously and we should hear some kind of an official statement from Canon soon.
As far as image quality issues with affected units, Imaging Resource had this to say:
The good news is that (a) the actual impact on photos is pretty small, and even then is limited to shots captured with very small apertures; and (b) it's quite easy to tell whether any given camera has flecks or not. I'd personally be comfortable buying either of these models myself, and either exchanging it with the retailer if it proved to have flecks, or waiting for what I expect will be an eventual repair program from Canon to deal with the issue. (Note that we have absolutely no direct information from Canon on this; this is just personal opinion at this point.)