Traditional TW player who doesnt understand War Hammer 3

against the human like factions I cna win easily but the monster factions like Nurgle, chaos and vampire fuckers, I just cant understand why I'm loosing.

Nurgle for example they will have infantry with similar stats or slightly lower than my best infantry adn they just steam roll my infantry then dispose of the rest.

My army is mostly gunners, halberdiers and swordsman with a couple outriders.

Any tips?