I play mainly duel, mirror, pangea (inland sea) games.
What I observed is that Darius is one the best player if not the best.
For ones who didn't play duel game, I want to highlight that the most important key is to keep balance between economy and military.
Darius' trait, unit, building maybe are not so obvious and strong as for example China or Greece,
but they complement each over, and you have access to it for all game.
Immortal - stronger than spear, you can built 4-8 of them then you can research civil service,
their trait unique ability is passed to upgrades units.
Sartrap Court (Bank)- not useful ?, very useful, in duel games military upkeep is the key issue.
Almost every city should have a bank (with golden age a few turns)
Unit moves +1, 10% stronger, GA 50% longer - this gives Dariusz huge advantage, with that
feature, you have unique ability to crush enemy army in few turns even having the
same army power, your unit can surround enemy, kill his general (a key unit),
GA bonus + Chitzen + Taj Mahal is awesome.
For single game etc. Dariusz should be also strong if properly managed