Spellcasting while in combat is also the most exhausting thing one can do (+5). It's more exhausting even than running (+4) and melee combat (+3). And the moment exhaustion ticks over, progressively more dire spell failure begins to apply. When your casting of Enervating Shadow fails due to exhaustion, the giant swarm of enemies you've probably attracted with your invincibility rips you to shreds.
It is very strong, but it's also difficult to manage. You've always got to be clicking. And your damage output isn't amazing, so you have to cast non-stop if you're using the Enervating Shadow invincibility strategy. And if you lag out for a few seconds, or mis-time your spellcasting clicks, you die.