It's a surprisingly fantastic language. In my opinion, it's the perfect language. But here are some issues in my eyes:

* The community is tiny. A rounding error compared to Go, Rust, etc.

* The core team of contributors is even smaller. You could comfortably have an intimate dinner party with all of them. If they lose interest, the language would likely die.

* The library distribution story wasn't great last I checked.

Caveat: I haven't used Nim for more than a year now. Maybe all these issues are no longer relevant. I really want it to succeed, but who knows what the future holds.