There's always the possibility that someone will design a 100 baseT device for USB 3.0. Really, you need both USB 3.0 (Superspeed) and 1000BaseT-- and you'll need to look at the specs to determine if either has been overlooked.
I got an Anker USB 3.0 Gigabit one from Amazon, which has the bonus of a 3 port USB 3.0 hub. It's hardly any slower than the Apple Thunderbolt to Ethernet adaptor, and then you'd have to peer at benchmark graphs to notice.