I can see that it wouldn't always be ideal, but on this (admittedly relatively niche) codebase - core telephone network infrastructure, which we only support on Ubuntu and which we build with GCC - the kind of clang/MSVC incompatibilities you talk about aren't a problem, and the reliability requirements are high enough that it's worth having CI check for 100% test coverage.

I think it's a question of field rather than philosophy - someone else in this thread said they do the same thing in avionics.