I don't believe typical developers use "plain POSIX environments," whatever that's supposed to mean.
What I mean is that there are init systems that only require POSIX compliant APIs and as such are portable across systems that implement POSIX, e.g. GNU/Linux, BSD, Solaris, AIX...
systemd requires (in addition to POSIX) Linux APIs and is therefore not particularly portable by any reasonable definition of the word. You can use it if you are running a Linux kernel, and that's it.
Those are irrelevant as they can't even do basic tasks like shutting down the computer or restarting it. Feel free to find a POSIX API for Linux' reboot syscall.
Some systems also require mounting additional filesystems, again not possible in a world limited to POSIX interfaces.
Sandboxing, SELinux, ...? Not in POSIX.
And so on.