I use ES6 at my job, daily. There are some things about it that make me really hate it. First, no, it doesn't introduce classical inheritance. It introduced syntax that
looks like classical inheritance. It's still prototypal inheritance under the hood. I'm not entirely sure what this means yet from the standpoint of building things with it, but I'm not enthused at the prospect.
What I find happens fairly frequently with ES6 that I never found with Vanilla JS is that the extra features built on top of JS 'break'. If you add syntax to a language, that syntax needs to work. I need to be able to rely on it working. I run into constant little issues that make me think I'm missing something about scoping, when really it's some under-the-hood issue with a library or something I just don't have the time to pin down.
One example is I tried using the spread operator to add keys to an object. But the spread simply, well, failed. Passing in the object worked fine. Passing in a spread version of the same object failed. I haven't resolved it yet, got pulled onto another feature. This sort of breakage is hard to Google, when I run into it again, and I'm sure I will, I may have to troubleshoot it all the way to Babel.
Error reporting in Node with ES6 is garbage. Worse than garbage, it's a veritable dumpster fire. Ideally the error points to the problem, when it doesn't point to the problem you have to rely on experience and intuition to lead you to the issue. Many many errors I come across in JS are of this sort.
I think most of these issues come down to the fact that ES6 is a transpiled language. This makes me long for the days of good old CoffeeScript. At least CS was close enough to Vanilla that it was easy to determine when you had an issue with the transpiler, simply grab the snippet of code and paste it into the online transpiler, look at the generated JS and work out your issue from there. It wasn't the smoothest workflow but it was effective.
ES6 is stupid in ways that make building an effective workflow unreasonably difficult. I can't wait to get back to Rails. Vanilla JS wasn't that bad. It was well-understood and you could work with it effectively on the front-end. It certainly wasn't as pleasant as Ruby, but it didn't feel like the pile of hacks that ES6 does. Maybe once it stops being transpiled it'll get better.