Bryan points out many weaknesses of unikernels, but assumes everybody needs multi-tenancy and the ability to spawn processes. And debugability is weak, but that probably depends on the application / environment you run. A lot of people want to run unikernels in a VM environment, but for me, it seems like the right application of unikernels is where you have one application that you want to expand to fill a single machine -- bare metal, boot to the application, save all the layers; there's no multi-tenancy, but if I'm running on hundreds/thousands of machines, I don't need multi-tenancy.

Debugability is important, but lots of people run without a kernel debugger, so it might not be that important to everyone. If you want it, you'll have to build it, but DTrace and friends had to be built too -- and it's easier to build it a second time, since you know it's possible and what it should look like.