Unfortunately not much more information than above. I was running a rather vanilla v17.04.4 installation with minimal configuration (just custom IP as far as I remember). I used the openwrt-18.06.0-rc1-ipq806x-netgear_r7800-squashfs-sysupgrade.bin package to upgrade, but the router just blinked after upgrade.
Using TFTP recovery I was able to flash openwrt-18.06.0-rc1-ipq806x-netgear_r7800-squashfs-factory.img, and the router works fine again. But it looks like there is something wrong with sysupgrade.
Upgrading from 17.01.x (or snapshots older than this ~june) to 18.06 or current master will require going through tftp recovery for ipq806x based netgear devices, as the NAND partitioning has been changed (the original kernel partition was too small and had to be enlarged to 4 MB, furthermore you'll now have around 70 MB at your disposal, instead of just 19 MB).
The bug mentioned by @LGA1150 will require you to use tftp again, to move in any direction; both issues are independent from each other, the former is unavoidable, the later an unfortunate bug that creot into -rc1.