The problem is whether the BIOS can properly handle the USB stick during the boot process, and the lifetime of the drive when it is left plugged in 24x7.
Experience has shown that USB2 drives tend to be more reliable than USB3 ones and USB2 ports more reliably than USB3 ones. Since Unraid runs from RAM other than a few seconds during the initial load there is no practical advantage to faster drives. There is also not much point in larger storage capacity as Unraid will not need (or use) it. It is also normally recommended that you do NOT go for the nano sized drives as they are more likely to suffer from heat issues in a server where they are left plugged in 24x7. USB3 drives are also prone to running hot. The 32GB limit is due to the fact that if formatted by the OS Windows and MacOS will format larger drives as vFAT rather than FAT32. The recommendation is normally go for a 8GB or 16GB USB2 drive as the best compromise between being big enough, price and trouble free operation.
I do not think there is much (if any) experience of USB-C and you are welcome to see if it works for you. Using such a high-spec drive does feel like a waste I would be interested to hear the results as going forward USB-C is going to become more common I would think.