When playing as Huns (I likely also for Mongols) the hint says food is gathered by combat and pillaging but pillaging does not yet seem to provide food to the nomadic army itself:

The amount of pillaged food also looks to be a bit too much considering how many is needed for the next unit (or the unit food requirement is tool low) - balancing question

Shenanigans with nomadic units during Lucy were well noted. The Devs briefly mentioned in their humankind day stream that changes were in the works for them. 

Each type of ransacked tile seemed to provide slightly different amounts of resources, but I don't believe they were too far off base. Sending in units to raid instead of occupy/conquer should be profitable. Armies engaged for conquest are less likely to raid as keeping quarters and infrastructures would make the captured territory useful quicker. 

Raids that yield food directly goes to ransacking unit(s) and I have seen armies of 1 horde turn into 4-5 more which is both awesome for the player but scary for the opponent. 

Sometimes ransacking takes a few turns... Zolobolo's 1 horde is going to take 12 turns to complete that ransack. Increasing the raiding party's Str decreases the time it takes to raid a tile which is where the Goth's LT shines. The amount of str needed for a 1-2 turn raid seems to depend on the era.