What matters day to day
Simplicity is the deciding factor. A device with one obvious button gets used; one that needs unlocking or navigating does not. Look for a large SOS button, a screen that is readable without glasses, and charging that is easy to manage, a dock is far more practical than a small plug.
Fall detection is the feature families value most, because it works when the wearer cannot press anything. It triggers automatically on a sudden impact and calls through your contact list. Check that alerts go to several people in sequence rather than to one number.
Related
For a smaller device carried rather than worn, see personal GPS trackers.
Common questions
Is it waterproof? Most are rated for showering, which matters because falls often happen in bathrooms. Check each product.
How long does the battery last? Typically two to five days depending on how often it reports.