It wouldn’t necessarily be a more difficult task to make a schedule in an open curriculum – indeed, with no general education constraints, it is likely easier, though some future slots might be held by placeholders (“whatever interesting free elective is offered”).
What can make the scheduling problem harder is if the student is undecided, particularly between majors without much or any overlap in introductory courses. Then the schedule must make progress in all of the possible majors of interest, at least until some are decided against, to avoid accidentally closing off the possibility of doing them.