Neat! I have tomorrow off so I'll give it a shot.
* 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.
I'm a C++ developer and, for now, sticking to it on larger projects since I love the thinking behind the C++ evolution over the decades. Rust still feels like a hype/moving target. The big selling point appears to be memory safety, but that's just a detail and not enough to justify a switch. I'm keeping an eye on how Rust and Nim will evolve in the next 5–10 years, but for now, C++, Erlang, and Lisp are my go-to choices for projects which should last the upcoming decades.
The safety is limited to data races, not concurrency or parallelism problems.