Cycling certainly has health benefits, but the general recommendation is to combine it with other forms of aerobic and resistance training rather than making it your sole form of exercise. Cycling only works a limited set of muscles, and doesn't generate the impacts or strains necessary to maintain high bone mineral density.
I read somewhere that you gotta bike twice as long as jogging to get the same benefit.
Also, stationary bikes are about half as effective as road biking. It seems that the necessity to maintain balance improves the results.
The same with weight machines. Lifting with barbells is more effective, and dumbbells even more so. This is because these require your muscle stabilizers to be engaged, while the machine stabilizes the weight for you.
That really depends on what type of benefits you're targeting. Do you want to get stronger? Increase aerobic fitness? Lose weight? Win a race?
Running and cycling (or most any other endurance sport) produce similar benefits for your energy systems as long as you hit the same heart rate zones. Most casual athletes find that for a given level of perceived exertion, running causes a slightly higher heart rate due to engaging more different muscles. But cycling is lower impact, so it's easier to sustain a target level for longer training sessions. Thus for general long-term health and fitness purposes it's better to cross train a mix of different sports.
Stationary bikes are as effective as road bikes for pure exercise purposes. It's just a matter of hitting the right power output and heart rate. Some pro triathletes now do the majority of their bike training indoors for safety and convenience. It's difficult to follow a structured workout on a road bike while navigating, dodging cars, and stopping at intersections. But with modern smart bike trainers it's easy to hit the exact target at each step and optimize physiological benefits. Some Zwift Academy participants have even scored pro cycling team contacts based on stationary bike workouts.
