The main issue I'm aware of is the lack of support for async/await in the various JavaScript parsers that devools uses. Some support is implemented, however:
- CodeMirror just got a commit for this: https://github.com/codemirror/CodeMirror/commit/b2c11167b61d20cbba14eaebde00a28f0a8e4bc5
- Acorn has a plugin to support async/await parsing https://github.com/MatAtBread/acorn-es7-plugin

In addition to these parsers, there needs to be integration testing done to ensure that stack traces and promise debugging features all work as expected in conjunction with async/await.

You can use async/await in Chrome in testing with --js-flags=--harmony-async-await .