It also depends hugely on the person. Riding fitness has almost no relation to running fitness.
There are tons of "weekend warrior" types who can ride a century every weekend but would collapse in a heap if they tried to run a mile in under 10 minutes.

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The problem with cycling as a cardio workout is that it's way too easy to take breaks without having to admit you're taking a break. With running, if you're running and not walking, you'll be at 70-80% MHR. If you want to take a break, you walk. Everyone sees you walk. You see yourself walk. You go more slowly. Your heart rate drops.
You're walking. You're going slow and you feel like a loser, so you wanna keep running. Keep going fast.

On a bike you can be cruising at 24 mph on the open road, and maybe when you are putting the hammer down you still hit that 80% MHR, but if you take it easy for 5-10 seconds your momentum still carries you along at that same speed. Maybe you drop down to 23.6 mph, but no one noticed, you don't look like you're working less, you still feel like you're going fast.
But you're slacking. And it's easier to do, because you're still going fast. So you do it more often.

I think cycling has a couple good niches in a cardio/weight loss regimen. One is for long duration, low intensity aka fat burning. It takes a special kind of person to run at a low intensity for 2 or 3 hours. Anyone can ride their bike for a few hours. Mostly because of the above: even when you're mailing it in, you still go fast, which is fun.

The second niche is in interval training. Ride up the hill, ride down the hill... ride up the hill, ride down the hill... ride up the hill, ride down the hill...
Disciplined people can accomplish the same thing on flat ground with sprints, but the thing about a hill is that you can see the top, and that motivation keeps you from lessening your intensity before you crest.
Plus, on a steep enough hill, you know that if you don't power through to the top you'll stall out and have to get off. That's an even more effective motivator than the walking-while-running-loser thing.
Fixed gear and single speed interval hill training is even more effective, because you don't have bail-out granny gears.