Your phrase already carries this contradiction: "__can__ be fairly expensive" points to something depending on what is being executed, rather than a property of the mode switch itself. Then, arguably, the same need of that costly execution should also be present with unikernels?
-- "context switch" (expensive) is the switch among active (running on this CPU) tasks/threads/processes.
-- switching between kernel and userspace would be "mode switch", much less expensive and of "constant" cost (see, for example, http://blog.tsunanet.net/2010/11/how-long-does-it-take-to-ma...).
Your phrase already carries this contradiction: "__can__ be fairly expensive" points to something depending on what is being executed, rather than a property of the mode switch itself. Then, arguably, the same need of that costly execution should also be present with unikernels?