If you have time, and someone hasn't already posted a bug report, could you please re-flash 18, then connect via serial, go through the sysupgrade steps again, and then post the boot log in the bug report to show where the error is at. If you don't have time, don't worry about it.
I may do so and post back here. The thing I just discovered is, that I've used the WRTSL54GS device as a WiFi <> Wired Client Bridge for so long, I actually forgot that the device crashes on WAN <> LAN running 17.01 as well.
This inability to downgrade issue has occurred to me on other devices before. For example, I cannot downgrade my Meraki MX60W to 17.01.4 from 18.06.