in general, luckily, nowadays even USB 2 devices can be as fast and reliable as FW devices. given that a company writes good drivers.
USB 2 still sucks in terms of power supply and different connectors. A MOTU Ultralite hybrid can be FW bus powered, but not via USB.
I am on a 828 mk3 and want to size down to a RME babyface pro. 10 years ago, I was on the pro-FW-party, since the old truths were intact. but the performance-reviews and the better compatiblity of the connectors give me the confidence to switch.
Even on USB 2, you can transfer ridiculous amounts of channels. RME decided not to go onto 3, since this would add no technical advantages but costs. Official quote:
USB 3 does NOT reduce latency at all. It just delivers more bandwidth to transfer more channels. RME can easily transfer up to 70 channels I/O over USB 2 (MADIface XT, MADIface USB). The Babyface has just 12 channels I/O. USB would have made it significantly more expensive due to a bigger FPGA and a special USB 3 phy required – for nothing.
USB 3.1 is suppose to be as good as TBolt2.
not in terms of speed. TB2 can combine the two 10 gbit/s lanes that were split on TB1, resulting in 20.
USB 3.1 is just 10.
TB 3 will be via USB-C cables and 40. but I bet that not every 3.1 connector and chipset will support TB, since this usually drives technical effort and cost up.