The AUR is a little different than a PPA. Yes, both are user generated content not officially supported by the project. However at least in the AUR there is a centralized place for these packages, and an entity behind the AUR platform that can curate bad/malicious/negligent packages out.
PPA's have a bad wrap for being, well... bad. Any user, anywhere. Here today, server gone tomorrow. Outdated blog posts from years ago with dead links, or bad advice/packages. It's not uncommon for a PPA to break a system.
The AUR is more similar to rpmfusion repo or epel repo (centralized and somewhat governed). Where PPA's are just like tarballs on some random-joe's blog.
Let's be honest; there are a ton of broken PKGBUILDs in the AUR, too. Lots of packages where a new version came out, upstream delete the old version, and the PKGBUILD hasn't been updated yet. I've also seen a few others broken for different reasons too... like some that don't include a function that's been mandatory for a while but used to be optional. I should probably get off my lazy ass and start posting comments and diffs on the AUR website.
Fortunately, hacking the PKGBUILD isn't a big deal.
Oh, and Arch also has its own third-party repos aside from the AUR. I've added a few for big packages that I don't want to have to recompile all the time, like Perl 6 and OpenSUSE's fork of Firefox. Also for ZFS, because it's easier to use the demz repo than coordinate kernel updates and rebuilds of zfs-git.
PPA's have a bad wrap for being, well... bad. Any user, anywhere. Here today, server gone tomorrow. Outdated blog posts from years ago with dead links, or bad advice/packages. It's not uncommon for a PPA to break a system.
The AUR is more similar to rpmfusion repo or epel repo (centralized and somewhat governed). Where PPA's are just like tarballs on some random-joe's blog.