Hi,
It seems well recognised that there's an issue with the pathing of NPCs through user created settlements.
Pathing (the paths an NPC can walk, or even where the cover is to hide behind) is usually coded into the maps of modern games. Often the creation tools of these games contain tools to auto-generate paths, but the auto-generated paths invariably require manual tweaking to get them right. A small gap between two objects may mean the auto-generator doesn't understand that an NPC could easily step over that gap.
So... that Fallout 4's NPC pathing with user-built settlements is bad is really unsurprising, and doesn't deserve all the bad rap it gets. To have auto-generated pathing working as well as it does is already an achievement, and I respect that Bethesda decided to do the user-generated settlements despite knowing (to make an assumption) that the pathing was always going to be an issue.
That said, I believe it's a problem with a possible fix. What if the game was able to learn paths from the player's movements? So, the player creates a weird and wonky bridge. The pathing engine can't find an unbroken path across it. The player then walks across the bridge without jumping or doing anything else an NPC can't do. The pathing engine notices this and uses the player's movements to re-evaluate the path across the bridge.
Just a thought, and I hope Beth reads this.