The obvious ones: Typescript + eslint + jest + Selenium. More complexity, high return on investment.
The non obvious ones: Without babel you are either writing legacy JavaScript (which is arguably not as clean and easy to read), or users will complain your site is not working on their older browser.
How old is legacy? I seem to do ok using most ES6 features. I still am not a fan of javascript, but the newer features don't do much. It's not really a readability thing, but the one JS feature I'm holding my breath for is native decimals. I could really use that.
The non obvious ones: Without babel you are either writing legacy JavaScript (which is arguably not as clean and easy to read), or users will complain your site is not working on their older browser.