As you know that every software of application comes with an Term & condition agreement and there is information about the Privacy issues also. Most of us downloaded Windows 10 and installed it, but almost many of them had didn't read the Privacy letters that comes along with the Windows 10.
Check the image below, and reads the [yellow] colored lines. You will come to know that what kind of data Microsoft is taking and sharing. It is clearly mentioned that the data they collect includes users name, email address, preferences and interests; location, browsing, search and file history; phone call and SMS data; device configuration and sensor data; voice, text and writing input; and application usage.
First of all by downloading and installing Windows 10, you give Microsoft very broad power to collect things you do (your activity) . They mentioned -
We will access, disclose and preserve personal data, including your content (such as the content of your emails, other private communications or files in private folders), when we have a good faith belief that doing so is necessary to protect our customers or enforce the terms governing the use of the services.Another nasty thing is that the data syncing is by default, where Microsoft sync settings and data by default with its servers. The data that sync includes - browser history, favorites and the websites you currently have open as well as saved app, website and mobile hotspot passwords and Wi-Fi network names and passwords.
Another worse things is that if a users encrypts the drive with Microsoft BitLocker then the encryption key aromatically gets backed-up to users OneDrive account.
So its your choice what to do?