Jinja is a text templating language. A valid ninja template may not produce valid JSON/YAML/etc. Sometimes you need more power than a config language gives you, specifically when you want reuse. Helm uses something like jinja, but the template files are often complex and messy (and they are looking into supporting Lua to deal with these problems). CloudFormation built function semantics on top of JSON/YAML twice: once in the form of templates and again in the form of its intrinsic functions. Skycfg uses the starlark language to output YAML, and this is the best approach since it has the right semantics and uses a familiar language (starlark is stripped down Python).

Why don't you take a look at the link? It has a page specifically addressing these questions: https://jsonnet.org/articles/comparisons.html

Jsonnet is much easier to make secure than JS. Untrusted JS needs to be run in a sandbox, can be insecure and contain side effects (e.g. read from the file system). Jsonnet can be statically guaranteed to not just be safe, but side-effect-free.