This card has got a laundry list of excellent abilities, but the cost is just too darn high.
It's great against Dack... but at six mana, it'll usually arrive too late to stop Dack from stealing an early-game Hangarback Walker or Foundary Inspector. Even when you're able to power it out early, I'd be awfully nervous about leaning on a six-mana artifact against a deck sporting Abrade and Ancient Grudge colors.
As pointed out above, the Foundry Inspector ability is nearly irrelevant once you're already able to pay 6 mana per turn through your own Spheres.
This card does excel at refilling your hand in the late game. Staff of Nin was replaced by Coercive Portal for this function, since the cheaper cost of the Portal was more important than the extra utility of the Staff. I do agree that Immortal Sun is an upgrade over Staff (not least because getting the Staff stolen by Dack is a huge blowout), but I'm not sure if the extra abilities are worth the two mana.