Both are incredibly powerful but in different ways
Flashback doesnt allow casting a card at any moment at will
The deck used the obviously-undercosted effect of Survival of the Fittest to put powerful creatures directly into the graveyard, only to reanimate them with Recurring Nightmare
In that case, Nettling Nuisance's ability triggers once for each player that Faeries you control dealt combat damage to, regardless of how many creatures were dealing that damage
Expensive, big-mana cards, ramp payoffs, or cards that care about having many lands in play are very interesting here
The term mill has been used over the years, across many different trading card games (TCGs) to refer to a strategy that involves winning by running your opponent out of cards in their deck