Systemd unit files can exec shell scripts…
There have been times where I needed to do something that wasn’t available in the unit file syntax, so I just do it in a shell script and ExecStart that.
I suppose, can you arbitrarily replace portions of systemd with scripts, though?
If so, perhaps it is less of a problem than it used to be.
It still doesn't fit every need but it's better if it can get out of the way when told to.
Configuring the system with textfiles only works if the system anticipated all your needs, and to anticipate all needs is not only complex but error prone.