I don't think there's a much nicer language when it comes to system configuration than a shell. I'm not sure why people want to write an extensive, complex program in a full-on programming language for simple tasks for which the shell and it's utilities were literally invented. The shell does a great job for managing your system already, and so far I've never had a better experience by making using a more complex language.
I've tried Ansible multiple times. Every time it seemed to cost more effort to do something simple, and once you start mixing OSs, things get much more complicated very quickly.
> It can stream shell commands so you've got all the usual sed awk etc
I can write a couple lines of yaml which includes the line of sed/awk/whatever, or just write the one line of sed/awk/whatever.
> And has a battle tested OS detection logic built in.
You can get pretty far with just checking /etc/os-release, and you'll need only a fraction of the code Ansible brings to do that. Ansible might do a _slightly_ better job, but the increased complexity is definitely not worth it.
> And can copy in different templates depending on OS.
Yes, it can do this, and I have done this. It's a lot of boilerplate YAML you'll need _every_ time you want to write any playbook. It's just not worth the ridiculous amount of hoop-jumping for something which seems to me to be a rather straightforward use case.
I've never used m4 directly, but it might be interesting to look at. I'll try to make some time to look around for information on this. If you have any pointers to get started, I'd be very interested.