I find IKEA Lack legs don't take screws too well, nothing to bite into other than the outside plastic coating. Better to drill straight through the legs and use nuts and bolts and large washers to spread the load. If the 19" items are a bit wide then there will also be some leg filing to do, which makes it less of an ideal solution.
I suspect IKEA may have changed the design and materials in the time period this has been on sale.