Smartphones do so many things for us, that many people are lost without them. They provide an assortment of tools: phone, contact book, day planner, calculator, instant messenger, and more. You can answer many questions by simply googling for the answer. You no longer have to spend wakeless nights trying to remember who was that actor in that movie.
The downside of smartphones is that they know too much about us. They will know everywhere you have been, who you have spoken to, and based on your searches possibly what you are thinking. Many smartphones are set so that they are always listening, so when you address a question to them they can respond.
The smartphones collected information can undermine your privacy, but at least you can keep track of where your phone is located.
However, much of your information is stored in the cloud. This means that you lose the fourth amendment protections against unreasonable searches and seizures, since you do not control this data.
Law enforcement agencies would have to tell you if they were to seize your phone, but they may be able to access your cloud stored data without you ever finding out. This is because the warrants are served where the server resides and they can be accompanied by a gag order telling the cloud hosting service that they are not allowed to inform anyone about the warrant. This police power has mostly been used for national security.
The issue with data residing in the cloud is that you no longer control where the data is located or who is looking at it. If a hacker or rogue employee accesses your data you will not know unless it is detected and the cloud company chooses to inform you.
To protect the data on your smartphone you probably should encrypt your phone and use a password or biometric identification, at least that will protect you should the phone fall into criminal hands. In addition to stopping logins to your phone, you need to make sure that you do not install apps that compromise your data. The best way to do this is to use popular apps from well known sources. Also keep your smartphone updated with the latest available software.
Another thing you can do is consider your smartphone a stranger. Don’t do things on your smartphone that you would not want a stranger to know or see. If you assume that everything on your smartphone can be viewed by a stranger and act accordingly you will be protected.