Support for OpenAPI 3.1 in .NET sucks. Up until recently, a third-party library (Swashbuckle) has been the heavyweight, but lagged for years in supporting new features & fixing old bugs. Microsoft created a first party option (Microsoft.AspNetCore.OpenApi) that supports 3.1, but it’s nowhere near feature parity with Swashbuckle yet.

I also find serialization/deserialization to be weak in .NET. Third-party Newtonsoft was king for years, then Microsoft released System.Text.Json. Years later, it lacks feature parity, including an easy way to debug like Newtonsoft did.