AUR refers both to a repository of upstream Arch Linux and Arch Linux ARM.
The latter is just providing binary packages of some of the packages in upstream AUR. Obviously these should work on Arch Linux ARM. Otherwise it's a bug.
Upstream AUR is providing Arch Build System (ABS) files that are used to build binary packages locally.
Very often, these simply fail to build on ARM devices as the architecture isn't stated in PKGBUILD's arch variable. This applies to
ProFTPD as well
$this->bbcode_second_pass_code('', '
arch=('x86_64' 'i686')
')Of course, the arch variable exists for a reason. Simply adding e. g. 'armv7h' doesn't make the software compile on ARM if it isn't suitable.
Regarding ProFTPD you may just try and build a binary package after changing the arch variable accordingly (unfortunately you didn't tell which device this is about).
If the binary package should work you may inform the AUR package maintainer and the arch variable can be adjusted in the AUR package.