It fucking sucks for handlock. 3-4 activations bring Stone to 4 mana, Molten would need 10 activations just like this to cost 0, with the difference that your opponent you getting low life .
I'm inclined to agree. Mountain Giant is bad late but good early. Molten Giant is bad early but good late and also good in situations where you're losing ground fast (exactly when you want to drop a 8/8 body for cheap).
Frost Giant is only good late and only in those late games that are very prolonged matches between two control-ish shells that's been activating their hero powers for 10-15 turns. It's not going to help you against aggro, it won't be as versatile as Molten against combo. It shines the most vs control, but not anymore than both Mountain and Molten already do. This is just another options if you want more 8/8 giants.
Consider, you'll need to have tapped 4 times and it must be at least turn 10 to combo it with Shadowflame. Unless the game slows down tremendously, that's not a luxury at most Handlocks' disposal. What will you cut for it? It's still not as good as Dr. Boom. It doesn't have the immediacy of Ragnaros. It's narrower than the other two giants. It doesn't combo with Voidcaller to replace Malganis, nor is it a pseudo-heal like Jaraxxus. It could concievably replace Sylvanas but then you're missing the Sylvanas-Shadowflame combo. Maybe it could replace Thaurissan? Anything cheap is irreplacable in the current meta. You need Healbots to offset the tapping. Slude Belchers to stop the aggro. Defenders to give taunt. Drakes to smooth out early game. Board clears, removals.
There's simply no room for it.