I have found portability bugs in many projects with slimcc just because it exposed different preprocessor defines, some gate critical __attribute__ behind __GNUC__ check, some have buggy fallback to __builtin functions or VLA that nobody noticed in years, these could have been avoided with just an automatic build job in the CI with kefir or slimcc, (tcc is awesome but less suited for being a drop-in).

It is also important to have more independent implementations of the C standard, not only to sort out dark corners in the specification (current WG14 have been doing great), but to prevent it turning into GCC-Clang power struggle.