While the new enemy variants are the most flashy feature of ABA, the AI improvements are at least as important to create a better tactical gameplay.
The AI used by the unmodded game doesn’t really play to win. Many choices are artificially dumbed down, something that stands in the way of satisfying gameplay that challenges the player. A Better ADVENT overhauls large parts of the AI, from global parameters over movement and targeting routines to unit specific behavior trees. The result is an alien opponent that plays less predictable and more efficient, making a real effort at stopping the player from fulfilling his objectives.