In theory. In practice systemd's actually much more transitive since many distros have adopted it and systemd services tend to work cross-distro, so once you learn systemd you can somewhat confidently manage Fedora, RHEL, Ubuntu, Arch, SuSE, Nix etc.

sysvinit shell scripts often differed in subtle ways between distros and the particular distro implementation was rarely transportable without quite a bit of work.

> You can do almost everything with shell scripts.

This cuts both ways. I'd argue the problem with sysvinit was precisely the fact that you can do almost everything with shell scripts, since now you're executing a script as root that can do almost anything, depends heavily on the bash abilities of the author to not introduce subtle bugs, or break easily, (which varied heavily from script to script and distro to distro), and to not be a security nightmare.