> I think I get it but I'm not really sure what I'm gaining over exception types. With an intelligent use of exceptions I can easily specify the happy path and all the error paths separately which seems really nice to me, ...

Until your coworker comes along and accidentally refactors the code to skip the exception catching and it suddenly blows up prod.

With tagged unions you can't accidentally dereference to the underlying value without checking if it's actually proper data first.

Exceptions are significantly slower than normal control flow in C# (about 10,000 times slower). It's also pretty non-idiomatic in both C# and most other languages I've worked in to use exceptions instead of a switch statement or similar to handle an HTTP error code. Also there can be multiple possible non-error responses from an endpoint you need to differentiate between, and exceptions would make zero sense in that case.