An important part of the criticism is that the apps being proposed won't tell you who or where you had a contact, just that it occured anywhere. This leaves you

An important part of the criticism is that the apps being proposed won't tell you who or where you had a contact, just that it occured anywhere. This leaves you with no capacity to judge the actual danger of being infected. An app being used to augment practical contact tracing, for example by reminding you who you'd been in contact with, would be potentially useful, but that isn't what's proposed. Another way that such apps become useless is their voluntary nature. Ie that use of the app, and self-quarantine in response to an alert, are an individual's choice. Obviously, if you passed by a sick person in the grocery store, you do have a chance of being sick, and therefore quarantining is still useful. But most people will feel silly about quarantining themselves in response to such a low probability event. Having even a small fraction of people act on these alerts would therefore require enforced compliance of some sort. Note that I'm not per se advocating either that the app be less privacy aware, nor that compliance be enforced, just expanding on the source of the impotency schneier talks about.