I think that there is some truth to that. If you understand ES5 JS, then ES6 is not difficult (not to trivialize anyone’s struggles). It’s just different. And a learner asks themselves, “Hey I already have ways to do strings and functions, why do I have to learn a whole new way?” A lot of the push back is just frustration that a lot of it is new ways to do old things. But of course there are advantages.
But the great thing is that if you know ES5, then ES6 can be learned in small chunks. And most of the chunks are pretty small. And really I think it’s better just to get an overview and then learn what you need as you need it.
And of course, really, there is not “ES6” anymore in the sense that most people are using Babel or something like it so they’ve moved onto ES7 and ES8, and even ES9. There is not really “ES6” anymore, it’s just JS and should probably be taught that way.