Depending on how many bins you want to put in your rack, it's very easy to make a rack that wont sag. With wood, moisture will be an issue, and frankly if you use plywood the same thickness as melamine it will have the same sagging issues if you make the shelves too wide and dont support them in the middle. Also, sealing would be such a hassle, not to mention the toxic fumes wood sealer produces, you'd have to let it sit quite a while. Melamine ftw.