The American Council on Exercise (ACE) recently published a study evaluating the accuracy of wearable fitness trackers.
ACE-sponsored Research: Are Activity Trackers Accurate?
The study is a bit dated given that it included the Fitbit Ultra (you hardly see anyone wearing this model these days) and a BodyMedia product (BodyMedia was acquired by Jawbone in 2013). Also, the study was small (20 people total) but the researchers measured true caloric expenditure via a portable metabolic analyzer. I'd really like to get my hands on one of those metabolic analyzers someday.
What's the bottom line? Even first-generation wearable fitness trackers are accurate at measuring steps. Once you start getting into more complex movements (such as using an elliptical machine or doing other complex exercises) these tracking devices start showing their limitations.
So, don't be overly concerned about accuracy. Use these devices knowing that they have their own limitations, but they can be highly motivating if you use them consistently and engage in your social network as you compete to stay active.
You can read about the study here.