I'm sure a lot of work goes into it, but often features that are present in upstream clang seem to be removed or heavily delayed. Fom the perspective of a Linux "developer" whose software (C++ scientific tools) sometimes gets compiled with the Xcode clang++, I've seen (i.e. been told of by Apple users) all sorts of problems that don't happen with upstream clang or modern gcc. The fact that the versioning isn't coherent with upstream clang makes things even more confusing.