Busybox is modular, it can be built without pidof. I know for sure it's not present for DSM 2.x but I don't know after that. Maybe it was introduced with DSM 3.x but I think it's most likely DSM 4.x.
I'm all for using pidof if that's compatible with all versions of DSM 4.x and 5.x. Support for older versions of DSM is nice to have.

Gentoo uses start-stop-daemon although theirs doesn't come from busybox so it might be
more advanced. https://github.com/daktak/gentoo/blob/master/net-misc/pooler-cpuminer/files/minerd.initd

Does pidof always get the right pid? You said multiple process were launched so multiple pid.