I've had the opposite experience. System.Text.Json adheres extremely close to the RFC and barely handles any edge cases. Newtonsoft will happily serialize invalid json. Not that you want that but if you have a legacy system it's not worth migrating. James Newton-King is a Microsoft employee btw.

Isn't Text.json also faster at this point because it uses all the new span goodness and the like?

I'm pretty sure it's been faster for as long as it's existed.

Turns out it's easy to be fast if you don't have any features.

Even better: it has an optional source generator now! So, at compile time, C# can be generated. This can be at two levels: (1) just generate metadata so runtime reflection is not needed, or (2) also generate code that does the serialisation or deserialization itself.

Pretty cool!