I see this reason a lot but what are some actual examples of what is lacking in the .NET ecosystem vs. Java?

Large Apache ecosystem (Spark, Flink, Pinot) is completely missing and third-party SDKS (looking at you AWS) almost ALWAYS have worse SDKs. The java Kinesis consumer and producer libraries are amazing, the ones for C# are simple wrappers around the AWS APIs which means there's a few foot guns waiting for developers to run into, even if they should know better.

Dev tools. The debugger is something for example that Microsoft ostensibly keeps to their own products, and how they totally slaughtered omnisharp.

It killed my daily csharp vscode driver couple of years ago, only now catching back up somewhat, but still unusable for bigger solutions.

That move made me gravitate towards vscodium, and avoiding csharp where possible.

Microsoft's move only recently got more understandable to me, because Cursor and others basically stole vscode to establish their "empire".

I mean I haven't done much in c# recently, but few examples that c# ecosystem is the subpar in quality

Kafka client library sucks, I mean it was a nightmare to make it stable and there were a few of them.

Pdfbox library

And many other libraries. If you use c# Microsoft libraries only - then you are golden. outside of that its really bad.

At this point I switched to Rust.