A lot depend on implementation of the app (is more a concept, is like putting Clippy along some modern well-tought assistant and saying that everyone in every p

A lot depend on implementation of the app (is more a concept, is like putting Clippy along some modern well-tought assistant and saying that everyone in every possible situation are bad), and a lot on the stage of the pandemy on a country (or potentially delimited enough region). And also how easily/fast you can get tested and how that scales and how responsible is the average citizen. In US or Brazil, where the virus is very widespread, it will give you a lot of false alerts (and a few right ones). In countries where the cases are relatively few and spread apart, having good warnings about being close enough with some of those rare cases may be the key to avoid spreading it. Once exposed, you can go around up a lot of days without developing symptoms, an early warning could limit how you interact with others at the very least (and getting yourself tested if in doubt). But a bad implementation is not a reason to discard the concept, at least, not that alone.