An ancedote...
I was standing next to Bill Joy after a presentation (circa 1990) and someone asked a question. He answered that he was thinking about rewriting Sun's OS Kernel, as soon as he found the time...This was BSD4, I think. Definitely pre-Solaris.
Now, I'm not an engineer of any type, but to think, "I'm thinking about rewriting the Kernel..."
Is that complex, simplex or amazing?
Well, the kernel is simply another program. The performance hit if it's badly written is heavier than if it were userland, and segfaults become deadly inside there, but it's not "magic". It's like rewriting anything else, only with extra responsibility.