In the unmodded XCOM2 game, Advent serve as the defining enemies for the early and midgame. As the campaign goes on, they are being overshadowed more and more by the alien species. At some point, pods of Captains and Troopers lose all threat to the player and become pushovers.
A Better ADVENT was initially made to provide more tools to the Advent faction, giving them more combat roles and strengthening their impact on the mid to lategame. This was done by introducing a handful of Captain and Trooper variants that use XCOM equipments and tactics. Today, ABA includes variants for all kinds of Advent units, giving each of them impactful combat roles that scale up into the endgame.
The following are examples of new ADVENT units that are added to the roster:
Captains: The unmodded Captain has gotten a facelift, turning him into a superior Support unit for his Troopers. To that end, he will now use Flashbang grenades and Suppression in addition to his standard Mark ability.
Troopers: A variety of Trooper variants fleshes out the ADVENT forces. Among others, there are troopers with shotguns, troopers with miniguns and in the lategame there are even troopers with rocketlaunchers. These units provide the main variety to the ADVENT encounters, keeping them different and engaging throughout the whole campaign.
Robots: The Riot-Control MEC trades his missile launcher for a device that throws cluster flashbangs and smoke bombs with large radius, thus providing great support to its ADVENT squadmates. The annoying Medi Drone zips across the battlefield while stunning XCOM or healing the aliens or just generally being a pest.
Other ADVENT: Lancers with shotguns are the ultimate close quarters threat while Shieldbearers with sniper rifles are great at staying back and causing damage from a safe distance.