Simple questions have a one-to-one question to answer relationship so we can determine if they have actually answered the question. Complex questions have a one-to-many question to answer relationship so it is not simple to determine that.
Ideal work around is to write validate rule of their own and reference in question pages. If they are using SurveySection ,they can write validation in the activity and reference in the post activity of Flow action.
This can be taken as enhancement request and feed back item FDBK-13422 is raised for the same..