Speaking from experience, using MicroPython is trivial and often "good enough", as are implementing modules for MicroPython in C should they be needed (C isn't my first language by a long shot, but I managed to write a module for MicroPython with little problem).

Also, MicroPython has pretty much all the features of core Python whereas mJS appears to have pretty much all the useful features of JS removed. Why not just use C instead of mJS?

Finally, couching Mongoose in a way that criticises the efforts of others is a bit of a turn off for me. Give me positive reasons for using your product rather than bashing the people you believe are your competitors. The culture and community of a project is important - negativity such as this is a worrying signal.