Yes, except if they alter the factory image. In this case a new factory image has to be flashed before factory reset.

I haven’t tried on my ASUS router, yet. But there might be a serial port in one of the RJ45 sockets, as on my old netgear. Plug in your PC there and run the manufacturer’s flash utility.

This trick worked for me when a normal factory reset didn’t help getting back acces to the WebUI.