Never wrote RPM specs because I generally dislike RPM-based distributions (Fedora was a really bad experience when I tried it), but from a quick Google search they're very similar.
I kinda like the format at a glance, seems pretty comparable in terms of what you put in there. Definitely less painful than debhelper.
I guess one of the advantages of PKGBUILD is that they're essentially bash scripts that gets sourced by the tools, so they're incredibly simple and don't require parsing a custom format. You can:
#!/bin/sh
source PKGBUILD
prepare
build
check
package
That comes with disadvantages in that reading the PKGBUILD is inherently unsafe, and it was the cause of many concerns back in the days with tools like yaourt, which pretty much just blindly sourced it to get the variables out, which means immediate code execution just loading it from the AUR.