The drawback of GRBL is that currently the G-code command set is limited, but it has the basics and there are CAM post processors that can make up for the lacking commands. GRBL on an Arduino Uno can provide up to 30kHz stepping rate very reliably.
There are free user interfaces for host computers running Windows, Linux, Mac, stand alone options, etc. the GRBL firmware is free as are many user interfaces for the host computer. The absolute cheapest USB motion controller is GRBL, It can be implemented for the price of a clone Arduino Uno, About $5.
I am posting for others that may read this thread. I know the OP has already purchased a UC100, and by all accounts it is a fine piece of equipment, but it is far from the cheapest.
