The uptake process of fertilizers differ based on the form of the nutrient - e.g., nitrate nitrogen vs. ammoniacal nitrogen. This is not necessarily a function of straight vs. complex - a decision on the form needs to take into consideration the speed of availability and the crop.
For that matter, SSP provides P, S and Ca - so it is superior to providing P from a complex fertilizer.
The one benefit of complexes is the ease of application, but otherwise straight fertilizers are equally good, if not better.