I routinely go over a year between updating some old crufty systems - usually it's all just fine, excepting a need to update the keyring first.

Periodically, some upgrades of some packages do require manual interaction when configured in certain ways. However, this is usually a general issue with a change in packaging which isn't actually any more onerous for systems that hadn't been updated in a long time.

Now, it's true that every once in a blue moon something really big does change. The big one I remember was updating a system that had been shut down for some years, and the compression used by pacman had changed in the meantime. That one did require some self-imposed manual intervention :)