So how good is the defense vessel?
The defense vessel isn't alone in being great. The TEC Garda and Vasari Sentinel both work exceptionally well. The Garda is the toughest of the three, the Sentinel has a wicked special ability plus phase missiles, and the defense vessel has the best damage output of the three. Overall, though, they're all superb units and back in 1.18 it was the Vasari Sentinel that was squeeking out wins against massed Advent Illuminators.
Collectively, these units are referred to as Flak Frigates. They're low-cost, have exceptionally good durability, and their damage output is very nice. If you can get all their gun ports firing, there are very few units in the early-game that can match that kind of performance. They'll counter LRF, Scouts, Fighters, and in sufficient bulk even bombers. Flaks are easily the game's most solid and reliable unit.
The flak frigate is countered by light frigates, and en-mass a LF swarm versus a flak swarm will quickly turn into an outright slaughter. Even if the opponent just mixes in light frigates (or flaks of their own), another big problem with flaks shows up; they cannot specify targets for all their banks. Many of their attacks will strike the medium-type light frigates which takes very little damage. Against diversified fleets, this can substantially reduce the damage output of massed flaks. Against TEC, you run into another devil: the Hoshiko. Because flaks deal damage to multiple targets simultaneously, the hoshiko has a much easier time cleaning it up, and as a heavy-type unit are largely impervious to damage from flaks and can absorb lots of damage from them.
Overall, the flak frigate is an incredible unit with unrivaled performance, but held back by some key weaknesses. Keep in mind that both TEC and Vasari are capable of these flak tactics as well. The difference is that Advent lacks strong alternatives, forcing them into this fallback, while TEC and Vasari have many other options and likely won't pick massed flak (particularly against Advent, who has the strongest light frigate).
I have to warn u if advent lock planets on small maps by tier 2 and keep u pre-occupied they can have a nasty revenge for calling them weak when there econ is up and running.
Advent has a wicked game once they're mature, but the problem I have is that a good TEC player won't let you live to see it, and a Vasari player has an even scarier game-plan once they reach higher tech levels. Advent does have some options, and if you can get ahead economically that will help enormously. Unfortunately, there is no assurance you can do this, and the necessary Progenitor opener isn't very useful against Vasari once the military battle begins.
As I said, it's not unwinnable. I'm always a big fan of the underdog factions, and in the current version that's Advent. I still remember playing Vasari and TEC back in 1.181 and desperately fighting against the more powerful Advent, often squeeking out a win by superior tactics. Now those same tactics will absolutely steamroll Advent.