For my wife, the answer is simple: she just didn't like Paul's voice when singing mostly solo, whether with the Beatles (Yesterday, Long and Winding Road, ...), solo, or with Wings.
Make no mistake, Paul was brilliant in pulling together a song instrumentally. But his vocal range headed to the rafters at times. No one was there to balance him when solo or with Wings.
everybody has that one thing that just annoys the hell out of them...and for me it was this band and their music. They way they looked, their attitudes..and their ridiculous, poppy, awful songs. Just the whole thing made my skin crawl. Is what it is...