Flat decks are easier to construct, as they don't require plate rollers and lend themselves to rapid line welding techniques. Flat decks simplify deckhouse and superstructure interfaces, increase commonality in structural members and scantlings, and provide level surfaces for the stowage of cargo (i.e., for container stacks).
Flat decks are susceptible to sagging between frames. Water has a tendency to pool, particularly in these sagging areas. There is a loss of strength associated with flat decks, though the relative strength of modern steel minimizes this affect.