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.
Claiming coverage has anything to do with reliability is just fooling yourself, at least if it's done on line or branch level. Will your 100% line coverage catch the divide by zero bug in this function?
int foo(int x)
{
return 1 / (5 - x);
}
I do a lot of cross platform stuff, and just having warnings as errors (a good idea) means a lot of build fixing because clang and MSVC disagree on things. If I were writing code in MSVC and I had to deal with code coverage reports from clang breaking the build, I would be paralyzed.
"Code quality" isn't a thing that can be trivially measured, and in my experience people that fixate on certain metrics tend to miss the forest for the trees.