I've been spamming flaks lately.
With Heavy armor and around 50% more hp/shields per resources/credits cost, and only 3 supply, they are extremely useful imo.
They might be only good at DAMAGING lrm's, fighters, bombers. But they are good at tanking anything.
AND lrm's and squadrans are the deadliest units in the game against most unit types, including your cap ships. If you neutralize bombers and lrms your cap ships can do clean up.
At the very least, it scares people to see they are so outnumbered

I'm making half my fleets up of flaks now and it's proving very very effective, largely because almsot everyone spams lrms or assailants.
You'll note that Illuminators are better against defenders than assailants or javs are. Illuminators have anticap which is 100% damage to them, javs and assailants have antimedium which is 75%.(Someone said 25%, they are wrong.)
So far I like vasari's flaks the most, as they use phase missiles, but I haven't tried a defender spam yet.
Statistically, the Vasari's Junsurak Sentinel is the strongest strikecraft-defense frigate, but due to it's price and supply requirements, it's effectiveness is about equal to the Advent's Defense Vessel. The TEC Garda Flak has weaker weaponry but is more durable than the others.
Junsurak Sentinel:
Cost: 400
Metal: 50
Crystal: 30
Supply: 5
Shields: 550
Armor: 3.5
Hit Points: 875
DPS Per side vs VeryLight: 7.5 Area-Effect
DPS Per side vs Light : 5.0 Shield-Negating Area-Effect
Defense Vessel:
Cost: 350
Metal: 40
Crystal: 20
Supply: 3
Shields: 620
Armor: 3.5
Hit Points: 650
DPS Per side vs VeryLight: 8.2
DPS Per side vs Light : 5.5
Garda Flak Frigate:
Cost: 375
Metal: 40
Crystal: 25
Supply: 4
Shields: 450
Armor: 4.0
Hit Points: 900
DPS Per side vs VeryLight: 7.0
DPS Per side vs Light : 4.7
Yes, the Sentinel's DPS/Side is lower than the Defense Vessel, but it uses Charged Phase Missiles and can hit several strikecraft at once. Against LRMs, siege vessels, and scouts, the missiles can ignore shields while the Defense Vessel's lasers cannot.