The PPA landscape certainly has changed and things weren't nearly as centralized as they are now.

I don't think the AUR is necessarily more secure but in general the nature of how the AUR works does perhaps give some slight advantage. The suggested approach to the AUR is to download and inspect the PKGBUILD so you know how/what the package is doing opposed to a PPA typically being a precompiled binary. AUR helpers will also typically show recent comments, etc. so if a package was compromised or had issues you'd probably know as you're pulling it down.

Really though the AUR just tends to be more convenient and complete than finding random PPAs in my experience.