Ah, you are talking about systemd, wasn't aware of that... I imagine that to be much more complicated for many use cases. E.g. running a unit test (as I describe in the article) isn't something I would use systemd for. Setting up a path and a service seems more complicated than using entr, and it is probably also harder to get to the output as well.
I am not exactly sure what you mean by that... But the main advantage is that the command does not have to be executed manually everytime you change something. Instead entr recognizes when something changes and re-executes the command for you.
That's true, but the syntax is different then. In this blog post I cover fish, and I didn't intent to say that this cannot be done in other shells (and I think I never said so).