Custom scripts will be disable when you enter GTA Online. Many script mods for GTA V depend on you having Script Hook V. Script Hook V lets you use custom scripts.Next, there are a few modding tools you should have on hand: dll mods, so be especially careful when downloaded these.
Still, no matter where you download a mod from, look through the comments to make sure other players aren't reporting issues. Mods from and are among the safest, because the websites scan for malware as the mods are uploaded.