I am not the one to promote XML, far from it, but the tooling for it is superb. Like, recently, we had to connect our (Java) app to NetSuite web service. All it took is to point to their web service endpoint, and the tool created strongly-typed (Java) client classes and proxies. That is in Java, and it works with C#, too (Visual Studio). Provide the endpoint, VS reads the WSDL exposed by the service, and generates all the required classes and proxies. And at all times, you see what functions the web service is exposing, and what parameters you need to send, and what kind of response you will get.

Much better then your typical REST API.