Here's a head-scratcher: how can a fitness tracker accurately track your performance on an exercise bike? We're not up for suddenly moving the thing from wrist to ankle, thanks very much.
What you need in this instance is an ANT+ sensor. Support for these is common among real sporty wearables, but rare in the more casual Fitbit-style crowd. ANT+ is a comms standard (like Bluetooth), a way for trackers to talk to bike cadence sensors, chest strap HR sensors and the like.
One of the few real casual-style devices to support ANT+ goodies is the Garmin Vivofit, because Garmin puts it into virtually all of its wearables. What we want to see is ANT+ and Bluetooth support in all trackers, as that means compatibility with smartphones and sensors alike.