Like any small multihull, Mbuli can be capsized.  Obviously that wouldn't be good in open water.  

Plenty of light multihulls are sailed in deep water, and Mbuli could be, too.  It's really pretty good in waves.  But in such a light boat, safety is a matter of the crew's skill, not the boat's innate "seaworthiness."

My personal definition of "seaworthy," that overused and misunderstood term, is worth repeating:

Seaworthy.  Adjective.  In a boat or ship, the condition of being crewed by sailors competent to know the limitations of the boat and themselves.

There's a series of short videos showing an Mbuli built in New Zealand here.

Mbuli's web page