I did a quick test with a single Finger (i.e. two CAN ports) and the USB adapters we have here:
With the right one (the one with two ports) I couldn’t get the robot working. For some reason there was a connection issue when I started the software. I can’t tell what exactly the problem was.
I then tried using two of the single-port adapters (the left one). With this the robot was running without further changes. However, I noticed that in position control the robot was vibrating a bit, which does not happen when using the PCI cards. This suggests that there are indeed some delays in the communication. Whether they really are a problem probably depends on your desired control rate.
So as a bottom line: It may work via USB but it will probably be less reliable than when using the PCI cards.