It helps to understand their strengths:
Battlecruiser: There's only one, because this is really a Carrier with guns. Take +4 Wings and 2 Frigates, and you have:
6-Wing Token
Max 17AD +11AD, or 2x 11AD + 10AD
A 6/9 DR/CR + 1 Shield is decent survivability, which is good, because you'll probably keep this on the table until it dies.
Gunships: 2 Gunships is actually normal for a Squadron Size, so it's a common downside to them. At 30", you can put out 13 AD, or 2x 8 AD, which is really strong at that range. Or if you buy Decimator Beams, you can threaten to Crit any Tier 2 Squadron with 9AD at 30". If you synergize this with a Battleship or Battlecruiser getting in close with Counter-measures, you can potentially threaten to Disorder a squadron every turn, which makes them easy targets for boarding (especially if you brought Assault Craft for your Battlecruiser).
Countermeasures is mostly a defensive MAR, as you'll generally only use it to keep a squadron Disordered, but sometimes you can line everything up and either use Gunships or the Maurauder TAC Inducement to force a Disorder Check on something within the Countermeasures bubble. Disordering a Squadron before it activates can be subtly devestating.
All theory for me so far, of course. I haven't played them yet, but I will be building a Mercenary Fleet with Syndicate, Omnidyne, Traders, and Corsairs. The Syndicate Patrol box will likely form my Core Fleet, because Spartan was very smart and made it easy to buy boxes to upgrade to a Battle Fleet Core by simply buying the Battleship box. That's about 500 and 800 to fill out Core choices, which leaves room for other Mercenaries. Omnidyne is good; ARTs strip crew and cause more Disorder, Traders run swarms and are surprisingly good for abusing something Disordred, and Corsairs are good for Mines and Assaulting.
