I'm not aware of any toolchain limitations; making the kernel compile as C++ is doable with a few trivial patches, and the kernel already is heavily GCC-specific, which supports both C and C++ to the same level.
software is complex, and you can't be sure until you run it in mission critical production.
considering doesn't mean it will happen in near future, because toolchains/ecosystem is not there, the same is applicable for many other projects.