Only place I see chainrub is small ring, smallest cog. That's often a result of shorter chainstays that sees the chain rub on the inside of the big ring. No escaping that and as Shimano would point out, you shouldn't use small/small anyway, there are other options to get that gearing using the big ring with larger cogs. You cannot compare this with a system using bar-cons as typically a bike using these shifters would be a touring bike with much longer chainstays that might eliminate rub on inside of big rings.