After sysupgrade, my OpenWRT x86_64 (combined-squashfs) install wouldn't boot. After many hours, I realized the problem was something I noticed before: if the imagebuilder image is built using the "FILES=" option, the system will not boot. If you build with the same set of packages but without specifying FILES=, that image will boot (to defaults).
I noticed this with 17.01 and actually forgot. I'm planning to report this as a bug as I have thoroughly tested it. But, I thought I would throw it here, anyway.
Anybody else notice this?