We all know the story beats of Warhammer. Massive civil war cripples' important organizations and people. 10 millennia of slow technological decline and overwhelming religious zeal with a side order of innovation being near-forbidden. The technical state of the modern imperium is far inferior than Imperium before the Heresy, or even directly afterwards.

What I'd like to know is what does the the modern Imperium have that the old Imperium didn't have? Be it from melee weapon like chain weapons or force/power weapons, to vehicles like the Rogal Dorn tank or the Stormlord superheavy.

I'll break this into two sections:

1: After Cawl's birthday bash gifts to Guilliman after his nap and the Indominus Crusade

2: Before that and the Fall of Cadia, when the Imperium was the worst position imaginable.