Might be a complexity issue. The PbtA games I've played (which is not a huge sample set, admittedly) were all much, much simpler than any FitD one I've encountered (again, not a huge sample) and derived most of their complexity from individual playbooks so players had less to deal with overall. FitD has far more interlocking subsystems and just generally more going on "under the hood" with its engine, which makes it harder for the designers to clearly express what their play styles expectations are - perhaps because they don't want to make you feel restricted to them. By comparison, PbtA games are usually very good at one specific, fairly narrow thing and as you noted, don't work as well when you get outside of that.