The only problem I have with this card is in its interaction with Momentum. Practically there is no defense against a quinted charger. The only options you have is to outdamage the adversary.
There are also many non-specific counters to immortality. Some examples:
Sundial prevents any creature from attacking.
Quintessence decks have low damage input, so they are easy to outheal with sanctuary and its like.
SoW Reflect is an entire deck based around making your opponent's creature immortal, then using Shard of Wisdom on it, and then having their attack bounce off of a reflective shield.
Shard of Wisdom complicates matters a lot, so I'll ignore it for now, but:
The thing to remember about quintessence-based decks is that they usually do not pack a punch in the offense department - sure, it's difficult to remove, but it's much, much easier to just unload as much damage as you can, and it's much easier to just work your way around it. (Without SoW) Immortality-based decks are simply just not fast. And immortal-based decks are not quintessence-based.
No, quintessence is much better used to protect a key component of a deck, and even then, you don't see much use for it.
Back to Shard of Wisdom... Yes, Immortal-SoW is a nasty blighter of a monoaether... But it uses Immortals, not quintessence. No one does quintessence THEN SoW for this deck type - they only use cards with built in immortality.
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TL;DR: Quint just slows everything down which means you can just outdamage/outheal/sundial it to death
