I recently set up a small server with the Rock64 booting armbian stretch 5.59 stable directly from a 120 GB SSD attached via USB3. Booting is handled with U-Boot from ayufan on the SPI flash. The Rock64 is board revision 2.0, the power supply is the original Pine64 one for the Rock. Besides the SSD, there are no other peripherals connected.
The board often (3 out of 4 times) hangs when booting. In this case, all LEDs are lit, the orange network LED will flash and the board is not reachable by SSH and not pingable. My impression is, that it is stuck because the SSD either powered up too late or the USB connection is delayed. However, I cannot confirm my suspicion, because the board also does not output any HDMI signal to my monitor (even if it boots up correctly). If I cut power, re-power and give it another try, it will boot at some point and everything (except the HDMI) will work perfectly.
My issue seems to identical to the one described here for the ayufan build. It is noted in that thread, that an external power supply for the SSD might mitigate the problem. I did not have a chance to test this, yet. But I also hope that you guys have a better insight into this and might catch a bug in the armbian image.