I worked on an Ember project once, the first JS framework I ever used. It really frustrated me that I had it had to use Babel to convert the ES6 into ES5. For some reason, on my laptop, that took about 10 seconds, so every time I wanted to check if some new code worked as expected, there was this long delay. There were a lot of other issues I had with it too.
I love working with straight Javascript (ES5), jQuery and template engines like handlebars. I can design things in a way that works for the project and can quickly just refresh the browser to check my changes.
There really is something to be said about simplicity. I love getting Flask/Sinatra/Spark/microframeworkhere and then just rendering a server side template that gets manipulated client side by jQuery. It's hard to move away from that sometimes.
As far as conversion, I don't notice transcompilation times. I'm at Ember 2.8 and whatever other versions that were setup.
I love working with straight Javascript (ES5), jQuery and template engines like handlebars. I can design things in a way that works for the project and can quickly just refresh the browser to check my changes.