The standard Raspberry Pi file system is read-write filesystem which can have quite some SD card activity in the background. Therefore it is a risk to power off the Pi not using a controlled shutdown. In case of a sudden power failure the filesystem may become corrupted. There are two remedies: 1.) Use of an uninterrupted power supply (UPS) 2.) Change the file system to a read-only filesystem.

I successfully tried the read-only filesystem solution provided by Adafruit.