Price: Consider how much you're willing to spend on a scale. Just because a scale is expensive, doesn't mean it's the best option. There are inexpensive scales that can provide the same measurements.
Features: Determine if you're going to want various measurement modes such as for pregnancy, pets or children.
Accuracy: Smart scales aren't always accurate and the only way to get the most precise measurement is through a DEXA (or DXA) scan. Consult with a medical professional if you're interested in this data.
Electrical currents: If you wear a pacemaker or implanted medical device, make sure the scale offers an option to shut off electrical currents. Currents sent by scale are generally safe, but they can potentially interfere with the device and ruin it.
Weight limit: It's important to consider the weight capacity on a scale since you want to make sure it can handle various weights.
Body image: If you struggle with body image or are recovering from an eating disorder, consider discussing with your doctor if owning a smart scale is the right choice for you. It's also helpful to know that some scales don't show you your weight or have a similar mode setting.
Body image: If you'd like to sync up the scale to shared apps, make sure your app of choice is compatible with the scale.