The female (nut) portion of the coupling will split with very little stress making the coupling assembly as effective as a duct tape joint.
Malleable iron?
I've never seen a steel one split. In my experience, the threaded-in portion of one of the conduits snaps off. And I've never seen a malleable iron one split under normal stressing. Usually splits in diecast fittings are from overtightening. If they split under use, the design criteria was faulty.
I'll agree that in a steel to steel fitting comparison that a threadless compression coupling will stand up better to lateral deflection than a 3pc threaded, but the opposite for longitudinal stressing.
Comparing the steel version of one to the malleable iron version of the other is an unfairly biased evalution.