Using a Javscript dialect is a huge disadvantage in my opinion. I have worked with a similar dialect, and even if the language is pretty close to Javascript, a

Using a Javscript dialect is a huge disadvantage in my opinion. I have worked with a similar dialect, and even if the language is pretty close to Javascript, a huge issue is that it breaks all tools and editors you would usually use. You have to rely entirely on the developers of that dialect to supply the tools you need. And this situation only gets worse over time, as you're decoupled from the improvements of the language. Especially once there are a bunch of changes in the dialect that conflict with later developments of the parent language. My experience is also that there is inevitably more friction than you might expect when integrating libraries from the parent language. For me this would be an immediate dealbreaker, it's just not worth it to tie yourself to a small dialect like this.