Nucleo + arduino shield = correct
Arduino + arduino shield = correct
But not this :
Arduino uno + stm motor shield = wrong
Nucleo + stm motor shield = correct
Impossible : there are no Shield manufactured by Mbed.
You have not understood Mbed project.
Arduino makes money by selling boards (Micro + Officials shields).
The purpose of Mbed is not to sell boards but to help his customers to sell IC.
Mbed is ARM.
ARM sells intellectual property to his customers.
Its customers are Atmel, Freescale, Maxim, Nordic, NXP, STM, Renesas, Wiznet, and so on.
ARM Customers win money by selling IC.
Boards are demo-board used only to facilitate IC testing.
If you are a professional you'll never pay a board, it will be given.
MBed customers offer few board that accepts Arduino shields, but they do not make shield*s*.
There are society as Adafruit or Sparkfun doing shields.
Have a look on: