USB ports on a NAS are there for a reason. You might occasionally need to plug in some stuff, like peripherals, or maybe even charge a device. Using it as a method for expanding storage, though?
If anything, it's a terrible idea to use USB-powered drives as conventional storage pools ...
Are you talking about using the USB function on the Synology? Avoid it if possible. The results will be in some cases it is faster. In others about a day later you'll wish you went over the network ...