Plus very important thing is understanding or alliance system. It have big impact on how populations perform within empire and external diplomacy.
Also Character layer is another level of complexity, which in some cases require full understanding of your race social system in order to know your potential/chances is necessary to maximize your game output.
Some information that been stated already + spoiler of things i left to be revealed by players till now - Archei have several special things:
High Concealment spies - Deep Cover specialisation allowing to have early/by demand full intelligence on enemy.
Early and very powerful stealth device allowing sneaky grabs and involving planet stealing approach with hightly stealthy fleets/troop grabs.
All Admirals have guaranteed Ship Egnergy Savings - it tricky but exploitable thing which also involve fuel savings.
Insanely rapid construction speed (mainly on Ice planets, they are very picky species) , with additional characters specialization and special group (it's bit rare though) which if can improve this even further. General big pile of maintenance/construction bonuses aviability through characters.
Troop regenration specialization, great with rebeling planets/planets under constant pressure.
Hight aviability of additional espionage bonuses.
Ships size.
Nice purple cybernetic theme.
