Just a small correction here, George was usually tasked with the lower harmony which, in my opinion, is harder.
In the example of Nowhere Man, the middle harmony is the main melody line, sung by John. Paul is obviously going to go high and that left the lower harmony to George. Lower harmonies are harder to make out since they're not as audible as higher harmonies so imagine with all the screaming and no monitors and trying to stay in key and tune and ALSO not to mention he's paired up with Paul on one mic who is actually 2 harmonies above him. It's amazing he could do it at all.
Another bear of a harmony is Yes It Is. Again, with George taking the lower.