The concept is the same (given a definition file, build a package from a central repo, aka ports-like), but one of the features of PKGBUILD has is that it's easy to build an ad-hoc package outside the main tree. For example, you can download a bundle of PKGBUILD and `makepkg` in any directory to build a package.

In other ports-like build systems, this can be a bit more complicated. For example, MacPorts allows you to use a local repository, but it requires configuring that local repository in `/opt/local/etc/macports/sources.conf` and `portindex` it beforehand before MacPorts could pick it up. Some others don't support building out of the main tree at all.

Personally, out of all ports-like building systems, I like MacPorts' Portfile the most. It's similar to FreeBSD Ports' BSD Makefile (MacPorts was created by Jordan Hubbard who also co-created FreeBSD Ports) but using DSL via Tcl interp instead of being a shell script (POSIX shell in the case of Alpine's APKBUILD, bash in the case of others). From my experience, the syntax is very nice to work with, though you need to know a bit of Tcl for a non-trivial package.