One of the goal of microkernels is to get rid of a whole class of security issues. In some cases it may trade performance for that. In some cases you can get better performances, but you have to rewrite your software to take into account the microkernel.

Regarding Cosmopolitan, one issue is that different executables can't talk to each others efficiently because they are in separate unikernels. If they are running in the same unikernel then you have essentially zero security, unless your unikernel is actually running another kernel!

In other words, unikernels aren't great for consumer devices.