I literally let my server sit without upgrades for almost three years before realizing my autoupdater script had been failing me. The biggest issue was needing to manually download a statically linked version of pacman (found on the arch forums) to support the new mirror/signing features. Once the system pacman was upgraded, I just ran `pacman -Syu`, let it spin for a while, and rebooted, and it just worked. ¯\_(ツ)_/¯

The main problem you’ll have is that you’ll want to upgrade archlinux-keyring first. Beyond that, it wouldn’t surprise me at all, though it also wouldn’t surprise me if you did have problems. Early last year I updated a machine I hadn’t booted for 2½ years, and I messed up the procedure a bit and ended up rendering it unbootable (the way I did it, upgrades in the pacman hooks system meant they didn’t run properly, so mkinitcpio didn’t happen, so I couldn’t boot past grub), but when I came back to that machine almost a year later and fixed it up by booting from USB, chrooting into the local installation and I think just reinstalling the linux package as a convenient way to trigger the mkinitcpio hooks, then it was fine, and after just a manual archlinux-keyring update the year’s worth of updates were uneventful. (I think if I’d tried to do the whole three years’ worth in one go it might have failed, though, vague recollections of the .tar.xz transition or something.)

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 :)

It works well, at least for me. I haven't updated mine for many months.

If I have problems during the process, it's almost always a signing keys problem. Package verification fails because too much time's passed between updates: the keys stored on my computer are too old and don't work anymore. I made a script to refresh the keys and everything started working perfectly.

That's what I was thinking. Good luck getting all the random crap you have installed to update without choking on EOL'd packages, or various packages that have new config formats, or changes to the updater itself, or apps which only support version-at-a-time migrations (or otherwise don't include the full history needed to migrate)...

Broke for me after a month or so on one laptop. Had to stop whatever I was doing and learn a bit how pacman and PGP keys interact or something.

If you boot from USB to use an external pacman to upgrade you can mitigate almost all of the common problems people have with this.

Ubuntu LTS updates worked incredibly smooth, for last 3 release. I am surprised somehow.