I can never seem to get the Rock64 to boot consistently from the SSD. Sometimes it comes up, sometimes it just "hangs". About once out of every 6 or seven times can I SSH into it - this is regardless of distro whether it's Bionic or Stretch. This is totall after flashing it to allow for USB boot.
On the Pi, I was able to solve this 100% of the time by using a small microSD card with nothing but "bootcode.bin" and an empty file called "timeout" which tells the boot to hang for a bit while things "spin up" and the SSD (or HD) mass storage come available. Once I did this, all my Pi's