Android compatibility looks feasible since the kit will be open source [1]. That should open up the ResearchKit demographic quite a bit!

[1]: https://developer.apple.com/researchkit/

If a person lives in the States and is willing to subsidize, iPhones are about as cheap as a lot of other offerings.

Though I'd imagine if a study requires the use of these sensors, an Apple Watch may be cheaper than something from a medical device manufacturer.

True, though you can always loan equipment to participants.

This study uses iPads for cognitive training in patients with Schizophrenia:

http://woolleylab.ucsf.edu/forecast

Personal use of the iPad is one of the perks of participating.

Not sure how bad that is, though. After all a lot of studies just employ university students

Regardless of any new constraints, this new method of acquiring necessary data is an improvement on the current state of affairs.

I agree. You would have to have Android support to have a more representative selection. Some Android phones are cheap enough they could be purchased for participants as well.