It's a tough call between C# and Dart. But I think C# has the edge.
The good
Boring and predictable
Great support from Microsoft, with new language features on a regular basis
Concise, expressive syntax
LINQ (the built-in functional-style collection extension library) has everything you could ever want and makes you feel like a genius
First-party support for web APIs, no third-party framework required
Very fast for such a high-level language
The bad
Only portable in theory. In practicality you're locked into the Microsoft stack by convenience and defaults: .NET Framework, Visual Studio, SQL Server, Azure, Windows. Again, you can break out of this, but you'll be carving your own path through the forest.
Hot reloading is also purely theoretical. Most of the time it's a stop-and-recompile development cycle.
The configuration for .NET Framework apps is XML-based and it's a bit of a black box because the documentation is very spotty and frequently out of date. Messing around with config files is always a bad time. Contrast with Node.js, where everything is JavaScript and if something is configured a certain way, it's because you coded it that way. The difference is night and day.
Autoformatting/pretty-printing isn't a first-class feature and I haven't ever gotten it to work as well as Prettier works on the front end.
Occasionally you get fatal errors with no debugging information other than a hex dump (no, not even a stacktrace), and figuring them out can take days. I've never seen errors this opaque in any other runtime.
Visual Studio is much better than any other IDE for writing C#, but it's slow as hell even on a top-of-the-line development machine.
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It's a tough call between C# and Dart. But I think C# has the edge.
The good
The bad