You mean awesome? Because awesome is good.
I'm not sure what to tell you. There are different kinds of songs. There are your formula Verse, chorus, verse, chorus, bridge, chorus, outro songs and then there are ones that break from the normal pop formula and are just constructed differently. McCartney is great at both. I still think the song is very cohesive (actually it's one of my favorite solo McCartney tunes). Think of it more like acts in a very short story. They can be dramatically different, but they still flow together. You may not find it cohesive, but I imagine he was betting that most people would follow it through and enjoy it.
Think of it this way, if all songs were constructed the same way it would get pretty boring. Sometimes songwriters want to get out of the box