Yes.
A unikernel is equivalent to a process in a more traditional system. We usually don't secure parts of a process against other parts of the same process. We just start more processes.
> Surely we're getting into realms of serious performance issues if that's the case?
Why?
Are you suggesting running an entire virtualised kernel in place of a process is not going to introduce a performance penalty?
There might also be latencies introduced in IPC.
Go, read the old exokernel papers (see https://en.wikipedia.org/wiki/Exokernel#Bibliography, especially http://pdos.csail.mit.edu/exo/theses/engler/thesis.ps). They got nice performance improvements out of running their equivalent of unikernels. It's exactly because they can cut through all the layers of one-size-fits-all abstraction.
They also address IPC.
(This reminds me, I should go and re-read how they actually did IPC.)