It's good to know other platforms and languages so you can adequately answer, for example, whether or not those upsides of Node and React are worth the downsides. If you have nothing to compare to then what do you base your opinions on?

But, that being said, there are downsides to trying to work in more than one language/platform. The main one is that you only have so much brain capacity. You simply can't be an expert in every platform out there all at the same time. So if you have a language/platform that you know really well and is suited to task you're doing then you really aren't doing it wrong.