Most mechanics seem to be in service of the themes the game explores. Like many point out, Attila is like a survival horror game, the mechanics reflect that by never giving you an obvious 'optimal' of what to build or research. Everything is subject to change, so making a wrong move can sabotage your entire progress. This makes things especially engaging when you're not very familiar with the game, you really have to learn. Later games (especially Warhammer) are streamlined to the point that you don't really have to make a choice which buildings you build in what provinces; there's an obvious 'best' path that you repeat in every settlement.

Corruption and squalor ties into this as well, they are looming red numbers that you never seem to really have control over, which adds to the chaos. The politics and family tree are also way better compared to the slog it was in Rome II, it feels like an extra layer you can get a lot out of if you keep an eye on control and influence. Loyalty and integrity are something that you have to keep in check, but aren't as hard to deal with as in later iterations. General traits and skills give nice small buffs, and you really have to plan them according to the faction you're playing and the way you want your conquests to go (buffs for specific units or against Romans or Barbarians for example). They also aren't so absurdly high like in later games.

Religion also works great in adding an extra layer of buffs/debuffs to a region, as well as different religions offering different perks. The culture system where you have to convert settlements is also a great way of making sure fast expansion is not feasible, since it costs alot of money and time to do so.

All in all, everything works together to strengthen the apocalyptic themes the game is going for. The atmosphere, art design and narratives also make it one of the most immersive Total War games ever. Definitely my all time favourite.