Is usb relay board safe for high powers?
From looking at the diagram, there is likely no isolation on the USB board (I only see 1 USB converter chip and no optocoupolers). In the event of a short on the board, it could send the high voltage back to the USB host, or to other places. This could be unsafe. I wouldn't use this board to switch anything above 30V and it would be really unsafe to switch AC mains voltages with this board. 

They did use proper spacing for the relay traces however.