probably you installed something manually at some point, otherwise pacman does nothing wrong. Rarely you have to touch something manually because a mayor upgrade, but I think it has been like 4-6 times in 20 years for me.

If you want to install something manually in arch it is better to create a package first (if it is not already in aur). That way pacman can check for corruptions before installing anything

Pacman works when updating all packages simultaneously. However when updating only a requested package it can break by failing to pull in other packages which need to be updated simultaneously. For instance, pacman can break pacman and sudo by updating OpenSSL to a newer version than the currently installed version of pacman and sudo expect.

Do you mean "an update required manual intervention but I did not pay attention and as a result my system was broken later"? If not, do you have links to the bug reports?

Ubuntu’s release upgrade also corrupts my system, so it’s not just pacman

I have been using it for 10+ years and it hasn't corrupted things for me. It can't corrupt.

It could get a bit tricky if you update very rarely, like once a year, but not sure what else there is to do to break things. Please elaborate a bit more.

Pacman will happily break pacman and sudo without warning if the user requests updating openssl without upgrading the entire system. When updating a package it does not pull in all of the other packages which will break if not updated simultaneously.

Indeed, pacman does not know which library versions are necessary for any particular app version. And almost all of the applications are dynamically linked.

Archlinux repo holds only a single latest version. There are some community maintained repos that contain older versions (and those can be easily fetched through downgrade utility).

When updating Archlinux, upgrade everything and then reboot. This way it should not break.

Any chance your RAM has errors?