Gregory's Server@niutech if a site needs to support Opera Mini, then I agree it makes sense to transpile to ES5 for those users.
But the point I make in my article is that you shouldn't transpile to ES5 and then just *assume* your site fully works in Opera Mini—because my guess is it probably doesn't.
Opera Mini is a very particular browser, and supporting it requires manual testing.
@niutech also, as an FYI in case you were curious, Babel's preset-env plugin does not support Opera Mini, see: https://babeljs.io/docs/babel-preset-env#caveats