We installed OpenWrt for the first time yesterday. The basic functions are up and running. I would say the installation process was smooth.
add system image http://openwrt.jaru.eu.org/openwrt-18.06/targets/ramips/mt7621/openwrt-18.06-snapshot-r7911-f65330d27d-ramips-mt7621-ubnt-erx-sfp-initramfs-factory.tar
show system image
reboot
There are some issues left.
Seems like the 256MB of flash memory is not available.
I'm not sure what instructions you followed, but on the ER-X, two installs are actually needed...the first is merely a bootstrap to get a very watered-down but operational version of OpenWRT installed, then you install 'real' OpenWRT on top of that.
Also note that snapshot builds do not include luci and it must be installed separately.