The firmware does not allow to send key sequences where the modifier remains down during the whole sequence, it always insert an up event after each key stroke. In windows, this prevent to inject unicode characters or in fact any character using the Alt+Numpad method (see https://msdn.microsoft.com/en-us/goglobal/bb964658.aspx)
For example, entering the letter Q in notepad using the Alt+numpad method requires to type
ALT+081
and to send the following key sequence
down ALT
down KEYPAD_0
up KEYPAD_0
down KEYPAD_8
up KEYPAD_8
down KEYPAD_1
up KEYPAD_1
up ALT
which is not possible.
This feature would allows us to develop payload that are nearly independent of the keyboard layout.