Mods unavailable in game

Emelyn702​  One major reason is that if your Documents folder is inside the OneDrive folder, OneDrive will try to sync the entire contents of Documents, including Sims 4 if it's installed there.  The game is much larger than the allotment of cloud storage Microsoft offers for free, so players might be "encouraged" (or threatened with potential data loss) to buy more online storage that they don't need.

Not doing so can lock up the syncing process, possibly resulting in lost data and definitely causing errors as OneDrive tries to accommodate the files.  And OneDrive may block game updates because it's trying to control this data.  This can become a problem after the fact if someone doesn't have Documents in OneDrive but later accept's Windows's offer to back up their files after an OS update.  So just because it's not an issue now doesn't mean it will be fine forever.

The other major reason is that specifically having the Sims 4 program files mixed in with user data, so having both in Documents > Electronic Arts (where user data will always be stored), has a variety of consequences.  One is that a game update may wipe everything in the Sims folder that doesn't belong to the game itself, which means all user files like saves.

The EA App can block changes to the folder that contains the game's program files when the game isn't open, so you may not be able to add mods or other files.  Testing in a clean folder is a basic troubleshooting step, but if you try to rename or remove a Sims 4 folder that contains both user and program files, you might be blocked from doing so entirely, and if it works, you'd be prompted to reinstall the game.

There are various more minor issues too, but these are the big ones I see affect players on a regular basis.  And when saves are lost due to an update, they're actually gone, not just moved.