Arch Linux

What different effect did you see?

The main difference is in where the package gets built, as you are the root user not your own user, the cache will be in a strange place and some config files may be misplaced.

Also, running as root is dangerous as the aur uses random scripts made by strangers on the internet. (edit: as stated by OP, mkpackape refuses to run anyway) This can be very dangerous, even when not run as root. I've seen an ip logger next to a list of "people who can fuck themselves", fork bombs, and have heard of crypto miners being installed. All in large well used repos.

Cemu, nordvpn, certain browsers, and many more are not distributed by their owners

Giving these scripts that are often made by some random person root access is asking for damage. People could just put dd if=/dev/random of=/dev/sda inside it and boom, your drive is not only gone but you can't even recover your data

Edit: they are correct, yay will simply refuse to install an aur package using sudo, apologies for suggesting otherwise

Running any AUR helper (I recommend paru instead of yay but either is a good option) should always be done with a normal user and never with sudo.

This is because both the download of PKGBUILD as well as the download of the sources and the compilation must be done with a normal user. Doing that as root poses a HUGE risk to the integrity of the system, a bad PKGBUILD, a source code with errors, a script with a space where it shouldn't be and you can even break your system completely, you could even execute malicious code!

It is the AUR helpers themselves that will (eventually) ask you for a sudo authentication for when they need to do the final step of installing the package once it has been built.

Yay asks for your password if it needs it. So you can just type in 'yay' and read what says.

Also: I canrecommende usind Peru instead ;)