In effort to improve the security of the Android ecosystem, Google added a malware check to the boot process of the new Android "Nougat" operating system. Basically, if the operating system discovers malware, your device will no longer boot or will boot in a limited capacity.
The downside of this system is that even a single byte of data corruption could cause your phone to refuse to boot up, but Nougat also brings additional measures against data corruption.
Furthermore, it also means you need an unlocked bootloader to use custom ROMs.
On the Android Developer’s blog, the company explains that Android Nougat strictly enforces that boot check, giving you far more than a warning. The good news is that if your phone is infected with types of malware, your phone will refuse to boot or will boot in a limited capacity mode (presumably akin to safe mode). The bad news however, is that some non-malicious corruption of data could also mean that your phone will refuse to boot up…
Android has alerted about system integrity since Marshmallow, but starting with devices first shipping with Android 7.0, we require verified boot to be strictly enforcing. This means that a device with a corrupt boot image or verified partition will not boot or will boot in a limited capacity with user consent. Such strict checking, though, means that non-malicious data corruption, which previously would be less visible, could now start affecting process functionality more.