I've rarely if ever had problems with UA metal flights posting, but I have had the issues the op speaks of when rerouted on another airline and trying to get the original UA credit. I have always been able to sort it out, but sometimes it has taken 2-3 phone calls and sending in boarding passes. Even when you call the 1P/1K mileage plus account service center they have to get a service director involved to manually credit a flight it cannot be done by the front line agent. I've also talked to less frequent flyers who have difficulty because they don't keep their boarding passes necesarily. The carriers should not make it difficult to get this retro credit applied to an account. I remember too with the irregular ops other airline situations getting the letter from UA saying something like that they couldn't verify I was on the original flight, etc. etc. and I had to call MP about it.

I fully believe that most people don't track their miles like a typical person on FT does, and "most people" certainly won't write letters, send faxes, and make numerous phone calls. I have friends that if they accidentally forget to put their mileage number in they just forget it. Still other friends don't even bother signing up for miles.