I'm definitely keeping my hands down. IKEA BILLY are the ones we've inherited from the previous owners when moving in to the current apartment, and we had a handyman reinforce each one in the entire row with angle braces, and then screw them together so they don't flex independently, and then screw them to the wall - and only at that point we felt comfortable enough to start putting books into them.

We've also inherited a few spare elements for BILLYs, I think some doors and shelves, never unpacked. I believe the previous owner gave up at the idea of attaching them to the assembled bookshelves, and never bothered to return them to the store.