I agree with your points regarding tooling and dev support in general. ATS, in that regard, benefits from being a "frontend compiler" to GCC - all the tooling for GCC can be used equally well for ATS codebases. Developers need to know C well (not an issue with all the learning material available), and at least one language from ML family (OCaml, Standard ML, F#, Haskell to some extent) to quickly pick up the syntax and common recursive constructs. There are three great books [1] available online, and a collection of common system programming examples that demonstrate how data types and algorithms can leverage safe language features [2]

[1] http://www.ats-lang.org/Documents.html

[2] http://ats-lang.sourceforge.net/EXAMPLE/EFFECTIVATS/