At a high level, microkernels push as much as possible into userspace, and monolithic kernels keep drivers in kernel space

There are arguments for each e.g. a buggy driver can’t write into the memory space of another driver as easily in a micro kernel, however it’s running in the same security level as userspace code. People will make arguments for both sides of which is more secure

Monolithic kernels also tended to be more performant at the time, as you didn’t have to context switch between ring 0 and ring 1 in the CPU to perform driver calls - we also regularly share memory directly between drivers

These days pretty much all kernels have moved to a hybrid kernel, as neither a truly monolithic kernel nor a truly micro kernel works outside of theoretical debates