WCOJs guarantee an (asymptotic) upper bound on join complexity, but often with the right join plan and appropriate cardinality estimations, you can do much better than WCOJ.

The runtime of WCOJs algorithms are even more dependent on good cardinality estimation. For instance, in VAAT, the main difficulty to find an appropriate variable ordering, which relies on knowledge about cardinalities conditioned on particular variables having particular values. If you have the wrong ordering, you still achieve worst case optimal, but you could have done far better in some cases with other algorithms (e.g. Yannakakis algorithm for acyclic queries). And as far as I know, many DBMSes do not keep track of this type of conditional cardinality, so it is unlikely that existing WCOJ will be faster in practice.

The new hotness is "instance optimal" joins...