I'm running the Asrock Z370 ITX board and have been able to get my Firewire 400 devices working Via the Thunderbolt 3 on this board.
I'm using the Apple TB3 to TB2 adapter into the TB2 to FW800 adapter and FW800 to 400 cable. Now that I know it is works, I may get the OWC dock eventually. I'm running High Sierra. I had problems installing my Apogee ensemble on my MacBook Pro with High Sierra, so older versions of macOS may work better. I had to have my FW devices turned on before booting into macOS (or windows with my old Presonus firebox).
The Presonus didn't like the legacy 1394 drivers as it did on a Windows 10 PC. I was using with a Firewire card installed. Now its using an LSI 1394 driver, I believe.
I used someone else's guide to install High Sierra and it worked well. Now that I have a Time Machine back up of a working install, I might see how I go installing El Capitain as i know it works better with my devices.
I'm very happy my devices are working as my Apogee was expensive when I bought it and the iMac 5K, which I was using, started having image retention issues. So I decided to get rid of it before it got any worse and try a hackintosh build.
The versatility of this hardware is awesome its not perfect but its damn close to it. The iMac was a POS anyway, IMO.