It uses c++ for the sake of using c++ instead of leveraging the extreme bonuses c++ provides over using c for embedded. For example, it tends to use dynamic memory allocation heavily, doesn't make use of unique/shared pointer, doesn't use templates properly (if at all) in places it would be ideal (hence increasing code size and slowing things down).
Last I checked, it doesn't support cmake at all, and instead used an extremely complicated and conveluted build system/process.
Both of those combined made me run away and never look back. Another side thing as of late is it seems to be officially supported by arm, which means it will never run on other cores like risc-v. I am not interested in getting stuck with one platform when riscv (in my opinion) has incredible potential for microcontrollers and is seemingly right around the corner.