Webster’s 1913 has no entry under this spelling.
Shades of meaning
Fernald has no entry of its own for unhomogeneous; he lists it under heterogeneous.
Where he writes about it:
A substance may fail of being homogeneous and yet not be heterogeneous, in which case it is said to be non-homogeneous or unhomogeneous; a bar of iron that contains flaws, air-bubbles, etc., or for any other reason is not of uniform structure and density throughout, tho no foreign substance be mixed with the iron, is said to be non-homogeneous. — from heterogeneous