a) now your QA team needs to know JavaScript
b) method completion is crap when you don't know the language.
c) "You can setup a DSL in scala.js and you can get the best of both worlds" OMG You want to add Scala to the mix too? AND write your own DSL from scratch, and take on its maintenance to support your QA team's needs instead of leveraging an existing one?
your solution:
1. teach the QA team Javascript interaction with custom scala created dsl
2. have the dev team spend time creating a dsl
3. have the dev team devote some of its resources to maintaining the new DSL
their solution:
1. use english-like DSL much easier to learn than JS
2. that is already created
3. that someone else maintains
both solutions are going to have limitations in that what you want the DSL to do will be more than the DSL can _currently_ do.