You should explain why you're struggling,exactly. There's every chance you're interviewing great developers but either your process or understanding of how to evaluate people is poor.
I know that is sacrilege.... every employer believes with utter certainty that they know exactly how to pick great people, but I think that's often incorrect and arrogant.... i.e. YOU might be the problem.
The real problem for unikernels is that they are effectively irrelevant until the major clouds provide millisecond boot times along with appropriate pricing models that charge for execution on a millisecond level basis.
Developers love to write code, but unless unikernel developers start speaking up instead of writing code, the major clouds won't get on board and provide the needed fast boot times plus pricing, which makes all this unikernel code not a lot more than interesting personal / academic projects.
Questions of unikernel security are certainly academic if the cloud infrastructure doesn't run them in the most effective manner.
One guy likes doing things in way, other does not.
Ignore.