Treadmill running isn't going anywhere. It's a staple in our gyms and a handy tool for being able to measure and control for our physical activity.
However, there have been some claims that treadmill running may pose an increased risk for overuse injuries.
The idea is based on the fact that overuse injuries stem from repetitive stress to a particular tissue, and this stress is greater when the variance in the movement is decreased. By this, we're referring to the fact that treadmill running strides will typically always be the same length, at the same speed, and with the same impact force. In contrast, running outside will have a much greater variability in the types of strides being taken, with the body needing to learn to adjust and adapt to the subtlest change in terrain.
While this claim is cited by several sources, few of them use hard, researched evidence. We do, however, have evidence supporting the general principle of low variance resulting in increased tissue stress. As well, while treadmill running has been shown to lower the rate of stress fractures, specifically, the same study found it to be less beneficial to bone strengthening than outdoor running. This might translate to decreased overall beneficial training adaptations from treadmill running and, thus, and overall increased injury risk.
I feel I must apologize for the inconclusiveness here, but hopefully it's good food for thought. Don't let this change your training regiment if treadmill running is currently working for you, but if you suffer from running injuries, it may be worth trying a modification in surfaces. And of course, if you're training specifically for running a race, the best training for you will be found outside.