I’ve been a rower longer than a cyclist and can attest to being a good complimentary routine. I found that rowing was a great overall workout,, especially if done with proper form. Good core workout as well. The big difference for me is the length of the workout. I’m 70 and the most I would do on a rower is 45 mins and usually 30. I can ride the bike for a couple of hours and the heart rate differentials are noticeable. It’s nice weather here in New England so I’m almost all cycling and will up the rowing in the fall. Concept 2 is my choice and there are a lot of different workouts on their web site to break up the monotony as well as videos to use proper form.
Good luck.