Lots of hard realtime systems (e.g. on microcontrollers) use a RTOS where there is even no distinction between user and system mode, thus one cannot even talk about microkernels vs. monolithic. From a software engineering architectural perspective this is not a good design, but it is very fast (thus the time guarantees you can give are smaller) and sometimes easier to guarantee runtime guarantees. And it is easier to write a RTOS where all things run in system mode (which does not imply it is easier to debug bugs or to maintain).