Arch wiki page on reflector states that:
Make sure the resulting /etc/pacman.d/mirrorlist does not contain entries that you consider untrustworthy before syncing or updating with pacman.
The question is, how should I know if a mirror is trustworthy or not?
Reflector relies on ArchLinux mirror status but limit the list you will end up using based on options like the country. This can already limit the “risk” even though the mirrors only grant you access to the packages so it’s basically a list of URL.
The risk will be to install untrusted packages or use “Siglevel=Never” if you have allowed such things in your system. Similarly if you use AUR it’s highly recommended to check the PKGBUILD before installation.