Actually, several Nova bombs were created. A few of them were on Reach when the Covies invaded, and they took at least one with them when they left. One of them obliterated an entire Sangheili fleet at Joyous Exultation, and damn near made the planet uninhabitable.
I just looked it up and it specificly said "a handfull" of these, so I'm guessing anywhere from 5-10, tops. That's another reason it shouldn't be made, to accurately reflect Halo it'd have to be highly limiting, which I believe is damn hard to do in Sins. The only real ways to limit things is money, and fleet. Considering the nova bomb is a weapon, not a ship, it can't be limited by fleet. So only money. But as we all know in the late-game money is of no consequence, so by the end of a game the UNSC will just be spamming nova bombs left and right.
"Several well-placed Shivas" would have nowhere near that much power. As far as we know, it takes an entire Shiva to bring down the shields of a Covie ship, and a Shiva can take its target's and the nearby ships's shields down. However it CANNOT both bring down the shields and destroy the ship. So several Shivas cannot and will not do as much damage as a Nova. Add in the fact that a Nova has a much bigger blast radius than several well-placed Shivas, and you have one hell of a reason to use them in very special situations.
I was obviously exagerating, but in gameplay if you launched a dozen or so Shiva's, you could damn near wipe out any covie fleet in the grav well. And considering Shiva's are unlimited, and come on almost every ship, whats the point of a Nova?
So a Nova is DEFINITELY worth it in a lot of situations. And with a kamikaze ship sending it in, you can still get your money's worth in destroyed Covenant. Especially if they have a nice big fleet preparing to conquer one of your worlds.
A kamakazi ship would basicly be walking on the ideal of realism in the mod. The humans would NEVER go "Oh yeah, even though we're almost extinct, go commit suicide to kill the covies." And if for whatever stupid reason they were willing to degrade to suicide missions (Oh, and don't say a word about any spartans, those were special cases), it's still a retarded gameplay aspect.
Ultimately it comes down to gameplay, if the UNSC can get a weapon that wipes out a grav well and makes a planet uninhabitable, the covies will need one too. Superweapons like this are just a REALLY bad idea for the mod, they were so rare and only one was ever actually even used in the Halo universe, they really just need to be left out.
