TL;DR: Flak Frigates, given no specific attack orders, were fruitlessly attacking large, armoured targets and ignored missiles streaming past them.
In a recent game one of my planets was coming under repeated pirate attack, and I noticed that the Pirate Corsair's missiles were extremely effective at destroying orbital structures. So I decided to augment my defences with some Garda Flak Frigates (the description mentions they excel at intercepting strikecraft and missiles) however, during the next pirate wave, I noticed that the Flak Frigates were shooting exclusively at the Corsairs (against whom PD autocannon bullets just kinda bounce off) and not the missiles they were firing, so they were offering no protection to my structures.
Based on the Garda Flak Frigate's description and it's general purpose, I would expect it to prioritise missiles over heavy cruisers.
Given that I've seen Garda's behaving as expected in other situations, I expect this was to do with the small size of the gravity well bringing the Corsairs into weapons range but in that case I would still expect defence frigates to favour shooting down fighters and missiles over attacking large warships.
- The Gardas were part of a mixed fleet, alongside Light Frigates
- The fleet had been given no specific attack or movement orders, the AI was wholly in control of what the ships did
What I think would help
- Some tweak to the AI such that, in the absence of any specific attack orders, Gardas ignore anything larger than a strikecraft when strikecraft and missiles are in range.
- A passive ability on the Garda that can be toggled by the player to have it focus on strikecraft and/or missile defence (the advantage here is that they can then be in a mixed fleet but can be set not to attack whatever large target you tell the overall fleet to attack but continue concentrating on point defence).
This would obviously apply to the Advent and Vasari defence frigates as well.