> Like, running emerge -u @world on a regular basis

You can run emerge -u sys-kernel/whatever-kernel-u-use, maybe followed by `cd /usr/src/linux; make bzImage modules install modules_install...` well, probably you'd use genkernel or something like that, instead of hand-crafted scripts.

The point is: `emerge -u @world` can run into issues esp. if you customized a lot, it can't be automated fully, but I've never run into any issues with updating the kernel, and it can be automated.

It is not so hard to upgrade kernel, the issue is with the reboot you need do automate. Or with live patching, which doesn't seem encouraging, as you say.