I'm on Debian 9.4. I did try that. Here is the output:
sudo bash Downloads/test/unRAIDServer-6.5.3-x86_64/make_bootable_linux
INFO: make_bootable_linux v1.3
INFO: The following device appears to be the unRAID USB Flash drive: /dev/sdb
INFO: Temporarily mounting unRAID USB Flash drive to /tmp/UNRAID_TMP_MOUNT
Permit UEFI boot mode [Y/N]: y
INFO: unRAID USB Flash drive currently mounted to /tmp/UNRAID_TMP_MOUNT, copying temporary installer files to /tmp/UNRAID
To continue you may need to enter your admin password
INFO: Installing Syslinux bootloader on /dev/sdb1
sh: 1: mcopy: not found
INFO: Writing MBR on /dev/sdb
0+1 records in
0+1 records out
447 bytes copied, 0.000791486 s, 565 kB/s
INFO: the unRAID USB Flash drive is now bootable and may be ejected.
Then, when I insert the USB into the server, the server says
The server can successfully boot from a Debian 9 Live USB, and the boot priority is for the SanDisk USB (all others are disabled), and no hard drives are connected.