Originally Posted by Llanite
As an AP champ, I oppose this.
Not every champ has a hard CC (stun/fear) and even if they have one, it comes with a catch (low range/heavy cooldown/ skillshot/ no other escape) which mostly make that CC their only reliable escape.
That immunity is fine with Olaf, as he has no other gap closer/hard CC. Now bring in a Xin with knock, charge and CC immunity, how am I supposed to pull him off?
If this is ever considered, there has to be another item providing gap-maker/immunity to physic DMG to fill that gap, and even then the game is still no longer balance.
Theoretically, Zhonya's would be this item's foil.
It gives armor to help against the weilder of Mercurial Scimitar and allows the mage to become invulnerable for the duration of the MC CC immunity.
EDIT: I don't think you thought this, but just to be clear, Mercurial Scimitar would not prevent magic damage aside the MR in this suggestion.