Transitioning existing code to use modules is also not entirely straightforward, though probably no more problematic than introducing unity builds.
A "Unity build" really just means typing #include "foo.cpp" a few times. It's trivial.
Meanwhile, neither Clang nor GCC support standard library modules. They have only partial support for modules themselves. C++ module support is non-existent in almost all build systems. https://en.cppreference.com/w/cpp/compiler_support
The idea of C++ modules is great. It's badly needed. In practice I'm not sure if they're ever going to be genuinely functional and widespread. Which makes me sad. Toy projects don't count.
Yes, it's very late, but progress is being made.