It makes sense that it wouldn't be very good at Go or Rust since both are rather rare languages in the open source world that copilot is trained on. When I did my first go project a few years ago, I had the hardest time finding even basic examples like how to parse a json string. Rust is even newer and less used. But java is the 3rd most popular language for GitHub projects so I would think it would do better with Java.
There are over 50,000 and over 40,000 repositories just on GitHub that contain Go and Rust code, respectively. [1] Among them, some truly massive projects like Kubernetes for Go or Servo for Rust. I will freely admit your argument for new and/or obscure languages like Hare, but Go and Rust are not "rare" under any reasonable definition of the word.
The failure mode seemed similar in all three languages. If you were doing toy things, or writing boilerplate stuff, it did perfectly fine. If you were writing something that wasn't a slightly-modified copy of some code that already exists out there, it fell apart. I don't think the issue is the language in this case — Go and Rust are common enough, and it rarely had trouble with the syntax — I think it's that the model doesn't go very "deep", so it's able to reproduce common patterns with minor variations but is unable to conceptualise.
Java is ultimately worse. It's old. It's gone through A LOT of change over the years. How does it even tell what's good and bad? Java also has lots of convention over configuration and "magic" in most frameworks, which it doesn't exactly understand.
When I tried it I'd often have to go back to it and keep telling it to use a different way of doing things because the world moved on. By that time there wasn't much point.
I see praises by those that have never coded in a language. They try ChatGPT, see it produce "ok" output and call it a day. If that's where we're going the web will be even more bloated than Electron and everything will be 10x worse. It's like low code but even lower (in quality).
