I have heard that they are spying into your phone as there is no mining which can be done on the phone. I myself wont recommend to get involve in this PI network and risk your security/identity as we don't know what data they are monitoring on our phone.
It's something that is advisable to download into old phones knowing that there are risks like this. pi doesn't really have a value right now and as far as I can see the team is just keeping their project going and doesn't have a plan yet to conduct a crowdsale. Pi network is existing for more than a year and I believed that there are only a few miners in the early stage that are still mining on their application today because their token isn't tradable.