Whenver I try mods something ruins the gameplay for me, often even the most popular mods have something that is poorly balanced or some immersion ruin glitches. Last time I installed The Sims 3 mods with a lot of new options, the problem was...there was too much options, the menu was horribly designed and often you wouldn't know what these options did, it was a total clusterfuck. Modders don't care about the friendly interface, balance etc. they only want the mod to look "cool" but they lack polishing, also the biggest projects most of the time never get finished.
I know they're free but if a mod stands out from the game and doesn't fit in some way then it ruins the gameplay for me and I might as well not play with mods. With graphics mods for example I noticed that they have problem with proper contrast, their skies are fucked up and shadows are glitchy, not sure if it's problematic in all game mods but the ones I tested they had these problems.