Although the Billy bookcase resembles a painted wooden bookcase, it's actually made of particleboard instead of real, solid wood, which is a main reason why it sags.
Weaker and less durable than solid wood, this material doesn’t hold up very well in humid households. Stacking the shelves with heavy books will also cause them to bend and sag.
While this shouldn't be a major problem if most of your books are lighter, it’s important to keep in mind that issues could arise once you fill each shelf.