The advantage to Thunderbolt (if you're using Thunderbolt 3) is that we now have "PCIe via Thunderbolt" drivers.
You have to be running a Z170x motherboard (with USB-C connection) and Windows 10.
Performance will be similar to using a PCIe card.
Regarding USB not being up-to-par for audio:
The RME Fireface UFX is a excellent performer when connected via USB2.
- 30 channels of I/O
- Can operate glitch-free at a 48-sample ASIO buffer size (as long as your machine can sustain the load)
- Round-trip latency at 48-sample ASIO buffer size/44.1k is 4.3ms
Running "PCIe via Thunderbolt" (or a PCIe card), you can take the ASIO buffer size down to 32 or even 16-samples.
That results in extremely low round-trip latency (in the 2ms range).
This is the main advantage to Thunderbolt (or using PCIe card).
PCIe via Thunderbolt (or PCIe) does have many times the bandwith of USB2.
BUT... if your audio interface is nowhere close to saturating the USB2 bus, simply increasing bandwidth won't increase performance.
Liken the situation to your automobile. Your car may have a top speed of 120mph.
Doesn't matter if you drive on a two lane highway or an eight lane freeway, your top speed is 120mph.