There are several ways to make sure that the mods and custom content (CC) you install are safe for your game. Of course you can never be 100% sure, but by following these steps you can eliminate some potential problems.
1. Always remove all mods and CC when the game is patched. CC can mostly be reintroduced right away unless it's CC that is also modded (ads game features and not just a new look). Mods are trickier, you should always check with the mod's creator if they are compatible with the new patch before using them or try them out in a test file before adding them to your game.
2. Create a separate game folder to try out new mods and CC in. This can be done through doing a factory reset like in the sims 3. Rename the "The Sims 4" folder to "The Sims 4 -old" and open the game. The game will generate a new "The Sims 4" folder that you can rename, "The Sims 4 - test file". You can now switch between the two files by renaming the one you want to play "The Sims 4".
Now you have a safe place to test new mods and CC. If worst comes to worst and you download something that messes up your test file, you can safely delete it and create a new one, and best of all, you never got the nasty stuff close to your real game files where they can do damage.
3. Always install 1 thing at a time (or a few things at a time) in a "newly added" sub-folderOnce you have a test file you can start adding mods and CC to your game. But to learn what mods and CC are good and what mods and CC are broken you need to know what you added and when. I recommend you create a folder inside your mods folder where you add the latest mods/CC. It is then easy to remove them one by one if things are acting up and trouble shooting is way easier for you.
4. Evaluate the source of the mod- Have you heard good things about this modder from different sources on the web? Do you know someone personally that have used this mod without any trouble. Are there people commenting on the mod and have the mod been updated since it was released? These are all questions you can ask yourself when looking at mods. If the answer is yes to these question chances are the modder knows his/her craft fairly well and that the mods s/he provides are good quality that won't mess with your game.
- Also, when downloading mods the creator will often comment on how likely the need to update the mod with each patch is. This is so you can be aware that some mods are more sensitive to changes in the game than others. If a creator does not comment on how likely it is that you have to update the mod after a patch at all, I would call this an unreliable source for a mod. Most good modders are very aware of what will effect their products and warn their users if there is any chance their mod will need to be updated.
Tips for improved load time when you have a lot of cc
http://www.carls-sims-4-guide.com/forum/index.php?topic=28624.msg467183#msg467183