Dread of Night doesn't do the same things as Walking Ballista against Miracles. Dread of Night does nothing against Jace, and can be killed by Wear/Tear or Disenchant that is being brought in to hit our Pithing Needles / Sylvan Libraries.

Ballista + Academy Ruins / Volrath's Stronghold provides a source of inevitability that Miracles is threatened by. Same idea behind Eye of Ugin out of Cloudpost decks: Eye will search out a big idiot every turn for the rest of the game. The problem is that Dark Depths struggles to present inevitability without being vulnerable to the pressure that Miracles is forced to apply.

I have tested various inevitability plans in Depths before. Each plan, from Eye+Emrakul to Urza's Factory to Nephalia Drownyard, forces your Miracles opponent to be aggressive, to kill you first. The problem is that many of these plans either can't do anything against a fast Mentor or Jace that will kill you first, or by them time they can do something it's too late, usually due to high mana costs compared to the mana costs of Miracles' threats.

Ballista can be cast for 2 mana, 4 mana, 6 mana, or perhaps even 8 mana in the late game. You most likely will not have to spend your entire turn to present a Ballista in the late game, while you would need all of your mana to make a 2/2 from Urza's Factory. Ballista can shoot a Mentor or a Jace in the midgame, unlike many of the other inevitability engines.

For these reasons, I think Ballista + Ruins or Stronghold is a consideration for an inevitability plan. I haven't had time to test Ballista much except one time against Death and Taxes, which I have recorded on my Youtube channel. It is Round 2 of my most recent league. Ballista for 2 seemed pretty strong in the matchup.

Edit: Volrath's + Vampire Hexmage is effective against Jace, not denying that. Pretty powerful actually. But it doesn't do anything vs. a Mentor + spells.