In particular, it's invalid to say "U1 is twice as good as U2". For that matter, even if you don't like utility functions, this is suspicious in general: what does it mean to say "I would be twice as happy if I had a million dollars"?

This is what I immediately thought when I first read about the Repugnant Conclusion on Wikipedia, years ago before having ever heard of the VNM axioms or anything like that.