Most mods are well maintained by the creator. A lot of them are safe to use. However, here are a few tips when choosing mods.
1. Make sure the creator is active. Sometimes they go on hiatus or stop modding and their mods become obsolete. And sometimes you will find other modders updating their mods.
2. Make sure the modder has listed the updated versions to correspond with game patches.
3. Always read the creator notes to see if there are any glitches or bugs that have been identified and to see if there are any conflicts with other mods.
4. Make sure you have all parts of the mod. Sometimes you need to use a mod in conjunction with a mod from a different creator.