Crosses within a fertile family are predictable:  a high percentage of takes, a relatively large number of seeds, good germination, and offspring that are of the same type as the parents. 

Crosses between fertile families are for the adventuresome:  few takes, few seeds, poor germination without special care, but this is the path to breeding breakthroughs.

Others  Seedlings produced by a cross between two fertile families tend to be limitedly fertile.  Some consider them sterile, but I follow the old hybridizers' philosophy that a "sterile" iris is just one that hasn't yet found the right mate.