- There are extremely few jobs that recognise it. I'm attempting to learn C++ because of this.
- Documentation can be lacking as there isn't as much demand for it, or people with time to write it. That said, personal support in small communities can be great.
- Smaller library ecosystem.
- Survival of the language into the future is less certain without the financial support mainstream languages have.
I've used Haxe for years despite these points, it's a great language. A language is more than it's engineering though.
Mostly ecosystem and community support. There are a lot of interesting languages out there, but it's hard to do interesting things with them if they're missing support.
Zig might be in a good position here as it has very nice C interop, which lets you leverage the past 30 years of programming history, but it's still got a ways to go before it will be "ready for primetime" from the look of it.
As long as it's a good language, why do we care if it's mainstream or not?