From my experience, they must have figured out a way to mitigate the variations in the process.
My lab (LTI in nyc) uses refermas in c-41, e-6 and one other in B&W but I cant remember the manufacturer.

All of my roll (120-220) and sheets are even and consistent. They process hundreds of sheets and rolls a day.
Also most of my clients use them also. I drum scan allot of c-41 and e-6 film processed in D&D processors in sheets and rolls. They are very consistent.

I have had some variations in the past with other labs in nyc. They would not let me in back so i dont know what processors they where using.