My understanding is that the centralised model means the government will very quickly get a huge database of who meets whom, how often, and for how long. Even i

My understanding is that the centralised model means the government will very quickly get a huge database of who meets whom, how often, and for how long. Even if you don't couple this with location data, it's still a privacy nightmare. If you test positive for COVID19 the government would use this information to send out alerts to people you've recently been in contact with. With the de-centralised model, your device tracks who you've seen recently and when. The government would announce the IDs of devices owned by people that had tested positive and your device would tell you if you had encountered any of the positive devices. The centralised approach makes it much easier for the government to track compliance (e.g. we know 500 people should have come forward for testing, but only 200 did) - but again, complete privacy disaster.