Any form of position identification is prone to errors or misunderstanding. If you have to start spelling out what3words phonetically then it loses some of its advantages over other geo-location methods.
It also relies on an electronic device to tell you your position. I can look at a map and determine my position without any batteries (though I might not be able to relay that position to the emergency services 
).
I think the weakness in this is nothing to do with ambiguity but rather navigation. If I hear a Lat and Long and know my own I can easily assess whether the other person is N, S, E or W of me. I live in a village where our ancestors thought it would be nice to give every house a name rather than a number. The owners of the first house are forever helping delivery drivers find the right address.