Uno power via motor board

Firstly, I searched or tried to couldn't find what I needed and it's pretty simple "I think"

I have an UNO and an Arduino motor board to power a motor running 12 volts. I seem to be able to run both the UNO and motors using a 12 volt source plugged into the motor board.

Assuming this will be the only task and the power will always be connected correctly, is there any downside in doing this as it simplifies things for me. The 12 volt source is sufficient for both too.

Thanks.

This is not the way to do it.
You need a splitter cable ("Y") cable so that you can plug the 12Vdc into the External Power jack on the UNO.
Two separate cables would also work but the +12V for the UNO should be plugged into the External dc jack.

I seem to be able to run both the UNO and motors using a 12 volt source plugged into the motor board.

Without a photograph I don't know how this could be working unless you are using a SHIELD and failed to specify that. "A motor board" does not mean "SHIELD" to anyone because if it was a SHIELD, it would have been called a SHIELD, and not a motor board, therefore we understand you to mean a NON-SHIELD motor driver. If this is not the case you need to be more specific. If you meant a shield then the +12V jack on the shield is probably connected to the Vin on the UNO and it is the same as a separate cable without the cable. Please specify or post a photo.

no problem, but 12 volt is max as the heat in the voltage rwgulator is the limiter

@fishytom,
Is a motor shield ?

Assuming the DC input to the motor shield connects to Vin on the Arduino, it's fine. The Arduino is designed to be powered through the Vin pin, which is why the motor shield does that!