I can find few better way of discrediting unikernels than to claim this as their killer feature.
(The actual killer feature being their ability to reduce attack surface substantially by simply having fewer syscalls that can be abused.)
As for the syscall/security argument I find that to be one of the weakest security arguments for unikernels. Fewer syscalls unfortunately leads to a much smaller set of applications that can be ran. Even trivial hello world style of apps can use north of 30-40 different types of syscalls. Not saying you should keep adding more but spending time trying to reduce what's there is kind of a non-starter if you want adoption.
I'd wager a lot of those are irrelevant in a unikernel world. Some of those might be dealing with ttys, or setting terminal attributes, or other needless things.
Drew DeVault had a recent blogpost on this very subject -> https://drewdevault.com/2020/01/04/Slow.html
however, when we were adding support for various language runtimes I was surprised at how much (ab)use there was where certain calls were used in a very very different manner than what was intended - for instance one might wonder why we have limited pipe support if we don't have the notion of multiple processes - cause ruby uses it to communicate from the main vm thread to the user thread - it's little gems like this that you simply won't know until you try to support it
then there's the cases where you might support one type of syscall but there's another syscall exactly like it except it has one additional argument
Can you not use Seccomp for that?