@niutech my understanding is that due to the way Opera Mini compresses assets, it's never handled JavaScript sites well, regardless of what APIs they were using (which is partly why Babel has never supported Opera Mini in its preset-env plugin).
So if you need to support Opera Mini then you need to make sure your site works well without JavaScript in general (a build tool is still not going to help).
@philipwalton Yes, but Opera Mini at least partially supports ES5: https://caniuse.com/?compare=op_mini+all&compareCats=all so transpiling to ES5 still has some small benefit.