The whole point of mages is to be able to "pop" a high-priority target in one rotation of their spells during late-game. However, with the current shielding meta, this is nigh impossible to do, and Shadowflame won't be the answer either.
The issues this item faces right now is in either false advertisement or poor player understanding. The item was supposed to be the ap version of an anti-shield item, or rather, an ap version of Serpent's Fang. The question is not whether the item is better or worse than Serpent's but in the way the item actually works.
The issue we face is the fact that magic penetration works with taking health into consideration. That is, the percentages vary from 10-20 percent magic penetration for non-shielded champions, and 0-10 for shielded champions, as realistically you will be looking to burst down a squishy ad carry with less health, but with more shields from supports on them. In essence, you can expect the item, when all Is said and done, to give an extra 1.75 magic penetration.
The design of this item is bizarre - the best use case scenario for it is actually waiting for a shield to fall off on its own, or for someone else to pop it, and then dumping your whole kit into that target - as being 'recently shielded' will give you the maximum damage boost from the item. So the 'anti shielding' item incentives you to not actually break through shields which seems like the opposite of what it should be doing.