That is very important to realize that root phones are not to be trusted, you never know who has access to your phone, but also there are some apps that require to be launched in the background and these apps also can watch your activities.
OP has a better point here, with root access those apps will work undetectable, not just in the background, they will be able to monitor everything you have in your phone, contact, videos, bank details etc, I hope people will listen to this very important advice