> The alternative [to Augeas] would be search-and-replace style edits (not robust), custom implementations for every format (hard work to get right, especially without Augeas' infrastructure in place), or being limited to templating entire files.

Precisely. CFEngine (3.x, but also venerable 2.x) does it the former way, but there are templating engines (Template::Toolkit for Perl, on which I have built my cfgen, and, less convenient to control, Jinja2 for Python, which is the base for my current work with configs) that make it easy to do the latter way.

CFEngine 3.x adopted also templating at some point, I believe (I remember second-hand communication with CFE engineers that said our (my) templating gave them some ideas, but it was long ago and may be invalid anyway), but since I haven't used CFE 3.x, I can't tell much.

Apart from Augeas, I haven't seen the idea of working with configs that had such a solid foundation. It's such a shame the gains from having a semantically-valid configuration editing are so small against the cost of developing and understanding the grammar of config syntax compared to generating the said config from a template (at least that's what I think, that the gains are so small). I certainly would love to use Augeas-like tool in my daily work.