It's complexity makes it completely unusable for all but checking other results. The heat transfer literature is filled with expressions for the form factor between various simple geometries. As is clear from how we defined them, expressions for form factors assume that both the source and the receiver are constant. This is an assuption that has been made by the heat transfer community and carried over into graphics. Although this is fundamentally flawed, in practice it works fairly well.
All analytic methods assume there is no occlusion between the source and the receiver.
In practice this is an unreasonable assumption. Generally, the solution of this is to take the visibility term outside the integral, multiplying the resulting unoccluded form factor by the average visibility between the two objects. Because in practice, the source and the receiver are well separated . When they are well separated, the differential form factor is roughly constant between all pairs of points. If the differential form factor were constant, then these approximations would be accurate.