The IIF () function as used in SSRS Expression Builder evaluates BOTH the True and False part before returning its final results. But if one of those evalueations causes an error, the entire result is an error, even if that is NOT the result selected by the evaluation.
For Example, two fields A and B. A = 1, B = 0
Set up an Expression of:
=IIF(B=0, "Can't Do this math", A/B )
This will fail because A/B causes the error, even though you are explicitly asking for something else.
However, in SSAS, in the Calculation Designer, you could actually use this type of logic and know that should it encounter an instance of B = 0 then it will still funciton and return the proper result instead of an error.
Also, there is no BOL documentation for the IIF function in SSRS.