I don’t think it’s microkernels in general but their microkernel design which wants as much as possible in userspace. They want each component to have its own memory space. ZFS blurs the layers between filesystem and the volume management. This kinda bothers layers of abstraction model folks. And I assume combined with their posix like model it just sorta clashes with what they want to do. Not impossible to integrate, but they want something a little different.