[csvan]

In the case of Babel - performance. Even with all the fantastic work that has been done on it so far, good luck avoiding massive transpile times for non-trivial ES6 code bases. I imagine TypeScripts (and other) transpilers have the same issue.

[15155]

In a project with over 600 required js files, 25kloc in project, way, way more from npm: the whole compile takes around 25s.

An incremental compile takes ~1-2s.