I am not by any stretch of the imagination an IT security expert, but it's something I'm interested in and I follow several people who are experts.
I am not opposed in theory to the COVID tracker app, but the govt absolutely has not done enough to subject it to public scrutiny.

I have not downloaded the app and I don't intend to until & unless my concerns are assuaged by actual experts.
It's important to remember that it's not a binary choice between downloading now or downloading never; we can take the time to scrutinise it, and download it later.