Why are BSP brushes inefficient?

I keep seeing it mentioned that BSP brushes are “inefficient.” However, I can’t find a good reference to why that is.

I would understand how trying to build a 2000 poly rock in BSP would be inefficient, unless it got re-baked to a static mesh, but if I need a rectangular hallway, why is a BSP any less efficient than an inverted rectangular prism static mesh?

Is it rendering, or collision, or workflow, or what?