The UA app for android is generally excellent IMO, but I encountered an annoyance.

I tried booking a few flights on it today. Everything went fine until I went to enter my billing address with its Canadian postal code, which has letters and numbers. The interface wouldn't allow letters. I tried copying and pasting the postal code into the field, but the app got rid of the letters, so that was the end of that and I had to call in.

I've since remembered that there is a "US gas station" fix for Canadian credit cards, where you can enter the three numbers in the postal code and just add "00" to get five numbers, and that works for pay-at-pump transactions.
It is conceivable that this same fix would have worked on the UA app, but I didn't try it, and I found no documentation that suggested this possibility.