I don't see how "templating purposes" answers the question. I deploy services on VMs that need to parameterized and I can parameterize via templating a written-to-disk shell script that is then just simply executed in the unit... or I can have more dynamic parameterization and use environment variables and Environment= lines in the unit. The latter solution means the substitution is effectively happening in the same place as if they were inlined parameters, so I can't imagine a scenario when it wouldn't be workable.

Both you and OP haven't mentioned why you needed to do it inline as opposed to in an external script, though.