I’ve heard some friends express concerns about these being used for government surveillance. As a civil libertarian, I share your reservations.
I’ve also been a mobile developer for nearly a decade and have done a good deal of professional work around the location-tracking and proximity detection technologies that these rely on, so know firsthand what these apps can do.
My short take: if an app asks for access to your location data or contact list, granting that will give more info than you want to share. If it instead asks to use the privacy-preserving contact-tracing capabilities Apple and Google built into the latest versions of their operating systems, it cannot be used to track your location or any personally-identifiable information, and you can use it with confidence.
None of these have been released in the US yet, but they should be coming soon. I’ll share further info on them when they are.