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Paul Prescod links to my to Infinity and Beyond essay as an indication of the problems that SOAP toolkits have in talking to one another. He's looking at the wrong place. If you want evidence of problems we have in getting toolkits to talk to one another you need to look here instead. The point of the essay that he did link to is that you can't get 30 digits of precision for decimal numbers when you interop with .NET if .NET only supports 29. There is no magic in this world, and so as long as there is diversity, there will be edge cases to bite you in your posterior when you least expect it.
Scanning his other discussions on REST, it appears that he generally uses the term "SOAP" in a narrowly sense to mean "RPC style SOAP requests over HTTP with SOAP encoding". But I did see a few gems in there like
too much stuff...the SOAP spec could be split into three or four independent specs that would sink or swim on their own
SOAP is full of optional features. XML philosophy was to drive optional features out as much as possible (we failed once or twice but overall did a much better job of SOAP). Optional features are an interop nightmare.