Answer:
In the case of result combinations in which load cases have been superimposed additively with "+", there might be wrong results in the context of nonlinear objects (tension or compression members, support failure, and so on).
Example: A tension member is subjected to compression in a load case and therefore fails. The member results are indicated as zero in this load case. In another load case, the tension member can indeed be under tension, thus remaining in the structure and adopting the internal forces. Now, if the results of these two load cases are added by means of the result combination, the results are summed up from two different structural systems. This can be a problem.
In the case of nonlinear objects in the model, you should rather use load combinations.
