Spartan is great, and the firepower is so impressive.
8 lascannons is certainly a fair whack of firepower, but I’ve found that the Spartan suffers from many of the same problems as the Land Raider, only magnified. It clocks in at 500pts, which is an immense investment in a single model. That’s fine in 30k where coupled with a flare shield the damned things are borderline indestructible, but in 8th Ed a Spartan is about as hardy as a basic Knight, which is not good. You take the Spartan over a pair (or nearly a trio) of Lascannon Predators to make use of the Spartan’s transport capacity. But if you do take advantage of that capacity, you exacerbate the ‘all eggs in one basket’ syndrome the Spartan shares with the Land Raider. Load it up with a squad of Terminators and a couple of a Characters to make use of the 25 troop capacity you’re paying for and you’ve ventured north of a thousand points...
If you do decide to pack your paper basket with golden eggs, you run into the combat units in transports problem I’ve been banging on about in this thread. If you move your Spartan up to get your combat unit inside to where it needs to be and your opponent leaps forward to surround it, you’ve lost the game, period. It’s all the same problems as the Land Raider, only worse because of the larger points investment and potential troop capacity.
I think 8th Ed is just too lethal for units that used to trade on durability. Whereas Land Raiders and Spartans used to be indomitable behemoths, now they’re just another tank to get whittled down. The fact that many armies will reliably kill a Spartan in their first turn speaks volumes.