memoryview can store anything, but only access and display
the native format. The reason is that conversions in the manner
of the struct module are very expensive.

You can construct all sorts of formats that memoryview cannot
handle:

>>> x = array(1, "c")
>>> x
array(b'1', dtype='|S1')
>>> y = memoryview(x)
>>> 
>>> list(y)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NotImplementedError: memoryview: unsupported format 1s


I don't think that the message is obscure, since all these
facts are documented.