Single letter keyboard shortcuts work but they "harm" any other shortcut that starts with the same letter. When you type a single letter watch the Status Bar.
Revit recognizes the letter as the start of a "hotkey".
If you type a second letter and it matches a defined shortcut Revit "fires" the command. If you use the arrow keys you can "cycle" through all the commands that have keyboard shortcuts that use the first letter you typed. If you find one you like you press Space Bar or Enter to "fire" the command.
I find that a three letter combination works better without messing up shortcuts like single letter shortcuts do. I double up the first letter...like XXR or MMO...