Not sure I understand why in-memory joins are wrong by default, especially in a large system with more than one data store. Let's say you call UserService::bat

Not sure I understand why in-memory joins are wrong by default, especially in a large system with more than one data store. Let's say you call UserService::batchGetUsers(userIds) to get a list of users from a service backed by a MySQL DB, call WidgetService::widgetsForUser(userId) to get a list of a user's widgets from a separate service backed by a Redis cache and another MySQL DB, and return them. What's the problem here? Lack of transactions? Unnecessary fields sent down the wire? Something else?