I never have used something like this app with our three teen kids. However we proposed they'd share all their accounts with passwords with us. For safety sake but also to monitor them when something fishy seems to go on. We strongly stressed we didn't want to monitor them, and did want to provide them as much privacy as possible although within limits. All teen kids agreed with us when they were about 10, 11 years old, also allowing us to check their passwords regularly.
In 8 years time only once we decided it to be necessary to invade the privacy of one kid as talking with the kid didn't solve the situation. We went through his social media, email and some other media and indeed discovered a probable reason of his strange behaviour. At that point we stopped as we found enough to have a chat with our kid - indeed a topic difficult enough to bring under attention of your parents (and the other way around wasn't much easier either). We did find a solution to the situation and everything was back in control. The kids did never tell us they had for instance a secondary gmail account. And I can only applaud them to have done such things. I'd have done the same thing for sure.
Start to discuss online safety when your kids are young, starting at 9 or 10 or even younger. Part of it are the password agreements we made. If you're starting to discuss this when they're 14 or 15 you're probably out of luck. At that age many are less likely to give up their privacy for safety sake. And then you might have to rely on solutions like this app.
Our teen kids are grownup now and didn't run into severe situations and never complained about this agreement. With our remaining 9 year old kid we do discuss this regularly.