Why is there a conflation between "modern" and "unstable", and "conventional" and "solid"? I've seen plenty of conventional things that are not solid, and plenty of modern things that were far from unstable. Or maybe I have a different threshold for what modern is.
Beyond your parenthetical, what about Rust is unstable for you today? It would be interesting to me to hear that in order to see if the things that come to mind when hearing that are the same that you meant.
First of all, there's no spec, no stable ABI, etc etc etc. And the language is constantly adding new features and complexities.
In practical terms and in this case it probably doesn't matter, but that is what people are talking about when they say it's both modern and unstable. It's not entirely unreasonable.
Note that "unstable" doesn't necessarily mean "broken". It just means that the ecosystem is likely to have changed massively looking back at code written today from some theoretical vantage point 5 years in the future.
Beyond your parenthetical, what about Rust is unstable for you today? It would be interesting to me to hear that in order to see if the things that come to mind when hearing that are the same that you meant.