Anytime you rely on a third party to add things into a game, you're taking a risk. I don't use CC or mods in Sims 4, though I did use a couple of Twallan's mods in 3. Never used them in 2. I have used mods in other games. The thing is, you unless you know that creator and they have a really good reputation, I wouldn't try it. Even then, things can go wrong unintentionally. And like @Catmando830 said above, the creator can also go MIA and not be available to fix things. Keep in mind, some of those creators could be 13 year old kids or anyone who is just dabbling and doesn't have a clue.
On the other hand, there are long time simmers here who do create good stuff and have solid experience and reputations.
As far as downloading from the Gallery, I'd only go for those that say there is no CC in their lots or creations. It doesn't always mean they're telling the truth, and it doesn't always mean that if it doesn't say there's no CC that there is any. Theoretically, they're screened somewhat ofr it. It's a craphshoot, really. I have used lots from the gallery with good results. That's probably your best bet. But again, it's a risk and things have gotten through. Caveat Emptor.