I find it very annoying that periodically, with seemingly no reason, the App drops all of my "Special Rate" codes and I have to re-enter them, HUGE PITA for me.

Also, it seems that I often have to close the App and re-open it to get it to re-fresh my Account Activity, again another PITA tied to a related PITA which is having to monitor to make sure that Marriott posts all my stays.