since November 30, 2017 there is a USB Driver for High Sierra 10.13 http://download.steinberg.net/downloads_hardware/Yamaha_Steinberg_USB_Driver/Mac/1.9.10-2/YSUSB_V1910-2_Mac.zip
BUT: on one hand, it solved an issue I had for nearly a year (need to switch back an forth the sample rate each time I plugged in the UR22mkII)
this worked for 4 weeks or so
until the HighSierra Update 10.13.3 made it even worse: now the sometimes (around 1 of 4 USB plug-ins) the USB-Kernel-Modul or Subsystem of MacOSX gets screwed up, I have to reboot the MacBook to get this fixed each time.
so can not recommend it currently