Multi player, you need to play with people you trust. You can also play solo, and there is no risk there. You can even mix this, and open up your (backed up just in case) solo world to people. Also, if you are controlling the server you play on (you can!) then it can go offline when you stop playing. If you host an always alive server... can't do anything about humans but the NPCS will NOT raid you when you are NOT home.

As far as NPCs go, there are enemy raids on your homestead(s) but there are also plenty of oft repeated tactics here on how to prevent any real damage.
the key points of those include making a moat, raising the terrain (its indestructible to NPC) as a 'wall', taking over a small island to prevent spawns, and so on. Your epic builds CAN be safe but you have to be a little bit proactive to ensure this.