We had to build tooling to break out solutions into multiple working solutions for use in VS and then use the actual solution for the build system. This meant that people would often break the build by modifying their local solution but not understanding what was really going on in the central build.

Our cutoffs were way smaller than 50 projects and 10K source files, though! (We had a lot of generated code, which we eventually mostly switched from generating code to generating MSIL directly to save the IDE.)