Nobody ever says the AUR is safe. In fact they say specifically that it's not; for exactly the reasons you mention.
That's why it's the Arch USER Repository. You take your fate in your own hands when you choose to use it.
As for your comment about using a distro that has everything in the main repo? How so? Every flavour has software that isn't included in the main repos. For Arch based systems, that means either the AUR or Flatpaks. For Debian based systems, that means adding new repos to your sources, which is exactly as unsafe as the AUR in most cases, or using Flatpaks.
If you've ever added a repo on Ubuntu, than you've essentially used their version of an AUR. The end result is no different.