See Load game error for problems related to neighborhoods. Some mods can corrupt objects or even cause malfunctions in the game code, resulting in one or more of the neighborhoods disappearing, which requires the game to be re-installed completely.
That's why it is always good to take extra precautions and backup saved game data, like whole neighborhoods. However, improperly installed mods or mods that do not work correctly can result in corrupt data, saved game losses, and severe glitches. Mods, when they are programmed, installed and used correctly, can be safe to use without risking damage to game data or saved games.
