There's an issue I remembered that almost always forces me to reboot my machines.
On certain Linux based distros after upgrading the kernel you loose the ability to load kernel modules as they'll be installated in /lib/module-<whatever new version> instead of the currently running kernel.

For instance this is noticeable if you've been running a machine for a while without rebooting, during that time you've upgraded the kernel without anything out of the ordinary happening, then one day you plug an external device that needs a module and the module can't be loaded.

Thus, at every kernel update I've built the habit of rebooting.