Keep in mind I'm the 'Aspiring' Ressurrectionist for a reason folks....
When it comes to maneuverability, the greater your flock, the further you can move. In a standard deployment you're looking at 24" of open ground between you and your opponent, assuming both of you setup on the 6" line.
With objective based missions you want to get a minion with the action points into contact with the objective, so you need to transport or push a minion around. Several methods have come to light for me over the last few months. There's the belle slingshot and the dead rider two step, but both are relatively short ranged transports. They'll get you to the centre of the board, but what happens when you want to get into your opponents deployment zone in turn 1? Exactly how can night terrors help and how many do you need?
The night terror flock seems to work on the guiding principles that they are fast moving at 6" per walk, they're spirits and have flight so they don't really have to worry about terrain, and they can flock together as a zero action. Flock together is a 6" pulse zero action that pushes all friendly night terrors into base contact with the active model. It requires multiple activations to pull off, and you have to ensure the active member of that flock doesn't get beyond the pulse's range. Here's how it works.
Activation 1 - activate Night Terror, move 6, flock together then move a further 6.
Activation 2 - activate a second Night Terror, move 6 and if there are more Night Terrors to activate, then flock together, or move 12 and flock the original model into contact.
With two Night Terrors you can move 18" in two activations
With three you can move 18" and have a single ap available.
With four you can move 24" and have a single ap available.
With five you can move 30" and have a single ap available.
The larger your flock, the further you can move, but the more activations it'll take to get you where you're going. The more activations, the more obvious your strategy. And that does seem to be the downside.
The upsides seem to be that it's all happening in turn 1. If you can score objectives in turn 1 then you can turn your attention to other purposes in turn 2. The relative strength seems to be in the flocks survivability. With hard to kill and spirit, your Night Terrors are going to be harder than average to kill, and as your opponent shoots or casts near you, you can always move closer using the 'Attracted to Noise' ability.
Lastly, and often in conjunction with attracted to noise come a trio of actions that can render your opponent unable to retaliate. These come in the form of the spell 'night falls', which will debuff their ranged attacks and the triggers 'cold darkness' and 'blind' which buff a terror attack or temporarily blind them.