By far one of them the most promising and expensive point cost unit you will field is the Annihilator. It alone has constantly contributed to many of my Scourge Victories, and its extremely rare to lose it during the battle.
The Annihilator has two modes, the first is as an aircraft. It has an excellent movement of 18" a round to get into position on the tabletop in this mode. In general round one I am flying it in this mode, and the second round transitioning it into its second mode. The earlier you get set up, the better. Here is a pic of its stats in both modes.
In ground mode. The Annihilator has better armour protection, and its 6 damage points, really adds to its survivability. For this reason, and the fact that I its an artillery piece and not normally in the front lines of the battle, I like to put a very high level Commander in them to insure I can maintain a steady supply of command cards.
The Annihilator is an artillery unit, that uses a large high energy blast. Its range is 24" which gives you plenty of opportunity to hit a lot of the battle field. Because of the size and strength of the blast, you very quickly stop your opponents from bunching up, and if within range of a castling up PHR, it gives you a good weapon to pound down upon the walkers.
A couple things to think about with this guy. It has demolisher two. So clip the buildings you need to if the opportunity arises to get in some extra damage. The artillery is also Indirect. This means using Minders for line of sight will help your accuracy with the plasma bombard.
You might of noticed that the Annihilator has a move and fire of 0 and a firing arc of F. This does limit your facing, since if you rotate in Dzc, you are considered to have moved. This makes it so you cannot rotate and fire your plasma bombard. While for most units of the game, I greatly dislike MF 0 units, the scourge have an answer to it. First off a forward firing arc is not too bad of an arc.
What solves the arc issue, is that Scourge command cards have a relatively common card that fixes this. If I draw it, I hold onto it for this reason. The card is relentless advance, and it gives you a +3" move and fire. Its definitely worth using as the battlefield conditions change.
Overall the Annihilator adds a lot to your Scourge forces. It does lock up 175pts of your army, and because your HQ is already a hard 140pts, it makes the points hard to validate. However, I normally attach the Annihilator to my HQ battlegroup (its a support unit), and dropping an artillery round right into an area I am about to blast with my Desolator's Ion Storm can do quite a bit of damage to an area. The key is proper placement where you can fire from relative safety round after round.