The primary concern with using inertial sensors on a treadmill (or any other large, metallic equipment) is the magnetic disturbance caused by such equipment and picked up by the sensor's magnetometer. None of Mobility Lab v2's algorithms use the magnetometer, however, so this is not an issue.
There are, however, some considerations that may be relevant to your use of Mobility Lab on a treadmill. In particular, gait on a treadmill is quantitatively different than overground gait. Attached is an article that describes these effects (Hollman 2015). Note the quote is from the abstract.
"This demonstrates the importance of consideration of gait variability when using treadmills for research or clinical purposes. Treadmill training may induce invariant gait patterns, posing difficulty in translating locomotor skills gained on a treadmill to overground walking conditions."