This has carried over from ARMA 2.
This may not be a major issue for most people, but if you, like me, know every radio command by heart, then it is.
Basically, if you're fast at issuing commands, the current UI is a bottleneck for you.
You have to wait a little between commands.
Eg. If I want my units to stand up and relax, I would press (Note that the key to the left of digit 1 is ½ for me, which selects all units):
½,7,6,½,7,4
This is
ALL, Stand up
ALL, At ease
However, the UI does not respond to presses within like 200-300 ms from the last one.
So, after pressing ½,7,6 for stand up, you have to wait. If you don't, it will ignore ½ and 7 of your next command, and only registers the 4. Thus, you end up with no units selected in the Mount menu.
Having to wait 300+ ms between commands, drastically slows down commanding.