If the returned creature can't attack for any reason (such as being tapped or having come under your control that turn), then it doesn't attack
As The Warring Triad enters the battlefield, the number of cards in your graveyard will determine whether any replacement effects that affect creatures entering the battlefield apply to The Warring Triad
This is a very flexible ability, but it works particularly well with a board full of Saprolings, doubling it into a massive horde which, if left unchecked, will continue to double turn on turn, until it consumes the entire pod
Wizards of the Coast tends to make common cards more simplistic in terms of abilities and mechanics, that's not to say that common cards don't deserve a place in your deck, just that your Rare or higher cards will likely allow you more opportunities to gain the upper hand in battles, or turn the tides
While its not strong on its own, the thing that makes Scute Swarm so powerful is that it is rarely on its own, and it can get out of hand very quickly
Join me next week for my annual State of Design column