This is total BS.
USB ports are current-limited devices. They have automatic protection circuits that step in and shut the port off if the current draw exceeds the maximum allowed by the design spec.
What you said earlier about the port talking too fast is also BS if the client device is properly designed. The client device can tell the computer "Hey! I'm old and slow. You can't talk to me any faster than XXX". This is why USB is backwards compatible from 3 to 2 to 1.
The major reason that many devices function properly over serial connections and break oddly over USB connections is that the manufacturer of those devices cheaped out. Most legacy devices are serial devices. There'd be a considerable investment of money into re-working them from the ground up to utilize modern technologies. So, to try to sound like they aren't taking the easy way out, they include a serial to USB conversion chip in the product itself. It's basically an embedded version of the USB<>Serial adapter dongles. However, if you choose a crap chip and then poorly implement it, the results are what so many people see with cutters. Random drop outs, freezes, timing problems, etc, etc, etc. There are chipsets and implementations that work very well, such as those inside the Keyspan USA-19HS. A company could just as easily decide to embed a converter of that quality but most of them don't for cost reasons. I would posit that this is why you see very few people complaining about Graphtec, Roland, and Summa failing with USB connections and a lot of complaints about Cat-named-cutters not working with USB.
EDIT: I do think I should clarify that I agree that buying a Keyspan USA-19HS is a great idea for any cutter that may have a dodgy conversion chip in it. The 19HS is a GREAT adaptor and is priced reasonably.