I wrote three letters of complaints to United over 6 months, but got only replies to 2. When I chased the other one up, they said that as they had received so much correspondce (2 letters is surprisingly considered 'a lot' by United), that they had assumed the second letter was a duplicate.

So it seems they actually don't read the letters, just match the Mileage Plus numbers, look at the class of service, and send out the appropriate voucher.

I've not bothered to write for the past 6 months to hopefully change their view of me as a serial complainer.