In my opinion, socionics is inadequate as a predictor of romantic compatibility. It is quite a bit better than chance, which is remarkable in itself. However, it does not live up to the expectations many people acquire when studying the theory.
What exactly socionics suggests is highly debatable. Some socionists introduce subtypes and use them to further model intertype compatibility. Some socionists talk of the difference between functions and the "specific content of the functions," which depends upon culture and upbringing (a copout explanation, in my view). Some introduce levels of development or intellect which may influence compatibility. Nonetheless, the above picture represents what many or most people seem to think after beginning to study socionics. It often takes many years for the understanding above to gradually morph into something more like the following: