Es2018 all the way. Web workers, classes, Isomorphic code, generators, async await, etc with a splash of Babel Devs have never been happier. Everything just works and online documentation is Rich and plentiful.

Plus as I said, core logic works on both server and client and we have offline support as an added bonus.

No bundle but you use Babel? How does that work?

Babelified code sits in another directory, I hotswap it if I think they are not on a modern browser (IE or older version of mordern browsers). So any normal Joe using chrome or ff or edge will not get a bundle. But if I'm not sure, the backwards compatible version is sent instead. So I think I can claim that there's no bundle since most of my users don't get one.