It leads to an improbable, but still interesting, question about sampling bias. We may learn a ton about affluent people who can afford iphones.
(Not a critique of the work, which I think is awesome and has the potential to do great things. I just think it's an interesting angle to think about.)
If this works, recruitment costs will go way down & researchers could afford to give/loan people iOS devices who may not own one. Very excited about Research Kit.