The Tech 2 Anti-Air Flak Artillery are mid-level fixed-position anti-air defenses. Their Flak cannons are direct fire weapons, but the shells explode in mid-air, potentially damaging several aircraft at once. Flak artillery can be built on water or land. They are great for defending mass extractors from air raids, or really anything from air raids. They should comprise a plurality or majority of your AA defense unless you really love SAM launchers.
Tech 2 Anti-Air Flak Artillery is unmatched at destroying Tech 1 aircraft, taking them out in groups before they even reach their target. They also fare quite well against Tech 2 aircraft. Against Tech 3 gunships and bombers, they are incapable individually of dealing with this threat... they are simply unable to deal enough damage quickly enough to prevent T3 aircraft from completing their mission. If you build several Tech 2 Anti-Air Flak Artillery turrets with overlapping fields of fire and protect them with a shield generator, they are a formidable force against almost any enemy aircraft. Any base with SAM luanchers should keep Flak Artillery around; twice as many as there are SAM launchers is a good formula. The flak's higher rate of fire can finish off enemy aircraft and help avert the overkill that SAMs are prone to.
The Tech 2 Anti-Air Flak Artillery has trouble damaging the fastest air units in the game such as Tech 3 air superiority fighters or T3 spy planes. Build several SAM launchers to compensate for this deficiency.
The 4 Tech 2 Anti-Air Flak Artilleries all cost the same amount. While each has unique weaponry and inflicts a different amount of damage per second, their overall performance is very similar, however, the UEF flak has the most damage per second and health, though it is not much ahead the others. The Seraphim weapon by far has the least DPS, nearly 20 less than the others for the same cost.