I don't think it's really fair to compare writing unit files to being able to program in shell.

True, knowing the systemd unit options is not a reusable skill, but it's not a huge effort either: the syntax is trivial (INI-inspired, the same as .desktop files) and the options themselves are rather well documented.

And for sure, these options are immensely more reusable when writing a new unit file than mixing and matching shell snippets taken from different init script using different styles and conventions.

(Not to mention how subtly tricky programming in shell is even when you're doing trivial things, see http://www.dwheeler.com/essays/filenames-in-shell.html )