It is the perfect Morde item as it gives him pretty much instant passive stacking, AP and Attack Speed and CDR but it makes him squishy if you build it because it gives no HP. This is the main reason why I do not build it.
Reduce the AP the item gives, but give it HP like 250 or 300 HP. Reducing the AP won’t hurt Morde because he doesn’t rely on high AP numbers. Also Nashor Tooth is an item that rarely any champion builds. Yes Kai’sa builds it, but that is a problem.