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.
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.