Substitution groups mingle with object hierarchies

Lists of interchangeable elements Substitution groups make it possible to say "where you were expecting X, allow Y, Z, N, or O." This can simplify the creation of models, except that it
Adds complexity to processing Depending on how related (or not) the components of the substitution group are, programs may have to scramble to support content which appears in a variety of different forms. Most programmers build objects with object inheritance in mind, not with an "any of these potentially unrelated choices here" approach in mind.
Cuts across the hierarchy While sane developers use substitution groups carefully, their mechanisms are separate from the object inheritance models used elsewhere in WXS. This adds an extra layer of constraints and options which cut across the rest of the WXS model.
Trees, restriction Substitution groups may include trees of possible substitution, and have their own restriction method as well.