In my fairly limited experience, it seems that up till a few years ago most "usb" mills and similar devices were actually serial with a usb converter in the box. That conversion step has always been a buggy pain in the butt.
newer systems are natively usb and you can just stream g-code to the port. While it may be technically slightly more difficult to make a device that is native USB, the old image of buggy USB just isn't accurate any more.
I've seen a few systems that didn't natively support USB. While they would claim usb connectivity it was actually a USB to serial converter in the box. They were a mess to set up and I ultimately ended up just finding a machine that had serial (can be a hunt now days).
The usb part is not their main selling point, but I feel it is a fairly good feature to remind people of.
@Caleb: Really appreciate your efforts in bringing these start-up stories up!! Who knows....may be these guys will make it to the top one day. Pretty cool stuffs they are making...going though their website I see they have made & released the stepper motor driver card for the Arduino board, which enabled them to develop the G-code interpreter firmware and next they integrated the controller and drives in a board to come-up with the controller card itself. May be I don't understand the real advantage of using USB interface for receiving G-code yet, except for gaining higher speed & connectivity to the modern world. I also see the feature on https://www.synthetos.com/project/tinyg/:
"Networkable via RS485 to support motion peripherals and for networking multiple boards for multi-axis systems and for really interesting projects (up to 1000 stepper axes)"
I have been somewhat mystified by how long serial ports have held on in several application areas. I recently proposed a design using USB only to have a manager refuse to even consider it based on a previous bad experience that he had with the interface. USB certainly is not a simple as a serial port, but just the fact that serials are just about completely left behind on PCs should be the writing on the wall.
Does this device do realtime control over that port?