Bottom line, it’s best to use the power supply that comes with the Roku.
The USB output current for usb2 is limited to a maximum 500ma. For usb3 the maximum output current was increased to 900ma.
The short power cable that Roku provides encourages using the tv USB output.
Never had a problem powering the Roku Express 3930x from the tv USB port. Upgrading to the Roku Express 4K+ exposed an intermittent problem with one tv model. It was right on the edge of a cliff where the Roku would work fine or fail. Using the external Roku power supply cleared up the problem.