I think they have a point, you might (and should) evaluate it for each new package you install. But when you do a full system upgrade, are you telling me you'll review every AUR package again?
Most AUR helpers (well, the ones I've used at least, those being yay and pacaur) include the option to show a diff of PKGBUILD (and other provided files) for AUR package upgrades
Well I don't use any of those dirty helpers (now THAT'S crazy talk) so the AUR packages mostly get built on a separate schedule (whether fully manually or in CI) from running pacman -S.
I think only one or two non-mainline packages I depend on that get frequent updates and it matters.