I've worked on unikernels for several years, and I've recently had some insight into low-overhead emulation. Depending on your use-case it's possible that a tiny emulator will be less overhead than everything else for the construction/destruction part. So, imagine that you have a programmable web server and each request needs to be served quickly. Then, reducing the overheading of forking the master machine is everything. The cost of doing a few emulated instructions into systemcalls (which largely model what you already have to check normally), you are going to have lots of room before you hit a point where something heavier like a JIT-enabled emulator or rv8 which has millisecond-overhead.

I am working on such a thing now and it has a 6-microsecond overhead in a highly concurrent scenario (60k req/s). So, I think there are cases for everything.