The problem with user repos vs the AUR is that you're trusting the maintainer behind them instead of inspecting the PKGBUILD and fetched sources yourself.
The entire problem is that people weren't inspecting PKGBUILDs. At least with user repositories, you have someone who can build a reputation for trustworthy packages. There's a reason why every other distro does user repos.
https://wiki.archlinux.org/title/Unofficial_user_repositorie...
The problem with user repos vs the AUR is that you're trusting the maintainer behind them instead of inspecting the PKGBUILD and fetched sources yourself.