I haven't heard it either. I looked it up in a Webster's dictionary. I wonder if it's countable?
If it is, it would be a crime against humanity to use it in the plural. ;-)
b
PS I use both, but I'm not a typical British Eng speaker (having spent nearly half my life in an American corporation). For me,
amibuity is a quality [quite abstract] and
ambiguousness is more concrete. But the distinction - if there
is one - is not clear, and I can't think of a clear example