You can use the AUR on other branches if you are careful, its only certain packages, or the moon is just right. But the “Arch User Repository” is … for Arch, so updates and dependency expectations, et al, may reflect that to a (sharp) point, depending on any number of interacting variables. To be the closest to Arch in these terms, and thus largely avoid those issues, in Manjaro means using the Unstable Branch.
PS. @Aragorn for the keyring thing…
You can see it run down here:
[UNSTABLE] pacman keyring issues - keyring reset after reboot