At my previous job it was older browsers but my primary avoidance of ES6 is related to my open source project (msngr.js) where I want to ensure I support not only older browsers but also node.js 0.10. I could use ES6 and transpile with babel but I try not to do that when possible to keep the build process as simple as possible.

If there was a specific ES6 feature that I think would hugely benefit my productivity or if I was working on a project that could utilize ES6 in other ways I would certainly use it more.