There are two different formats for Custom Content: Sims3Packs and Package files.

Sims3Packs are installed through the Launcher. They are also uninstalled through the Launcher. Make sure you uninstall form the installed content tab and not only deleting through the downloads tab.

You can also convert Sims3Packs to Package files and install them the in your mods folder. Store content should not be installed this way because a potential Store Patch does not apply to those and there are extra measures which you would need to take.

Faulty Sims3Packs can be found with Custard but keep in mind that not all errors can be found with the tool.