Priceable units add to the complexity of ticket pricing in two ways. First, the choice of different partitions of fare components into priceable units increases the search space. Second, and more importantly, priceable units introduce long-distance dependencies between different parts of a trip. Notice in the case of the circle-trip priceable unit how the Saturday night stay restriction tests the combination of the departure times of two flights widely separated in both time and on the ticket. Nothing limits the distance between different fare components of a priceable-unit, which has huge ramifications for search algorithms: most efficient search techniques demand that constraints be local.
Fare rules can and usually do restrict fare combinations within a priceable unit as well as flight combinations. For example, a fare may require that other fares in the same PU be published by the same airline and have similar basis codes.