Good question.
For a constant HR - rowing is MUCH harder than cycling for me. If I row to a HR of 150, I'm right on the ragid edge, sweating profusely, and in the end it's unsustainable. If I cycle at 150, I can keep it up for hours - on road, or on turbo - and still be conversational.
Equally, what is a "Workout" on the rower (130-150bpm), is a "Warmup" on the turbo.