As a current user of the said item of furniture I'm curious what makes it really bad as a book shelf.

It's a shelf and in the past few years it has not yet failed at holding my books.

It depends on the loading. If you've got it 9-12" high apart and are putting paperbacks on there... that shouldn't be a problem.

However, when the shelf becomes multiples of that, then people start putting hardcover volumes or laying the book flat and stacking them high within the shelf.

It won't necessarily fail, but it can substantially sag with heavier loads.

This is an issue for boardgammers who are after larger and heavier shelf spaces which is why the Kallax shelving is much preferred. I'll also note that Ivar shelving is solid wood rather than particleboard.

The shelves will start to sag substantially if you fill them.