Thought Scour puts a random card into your hand, Opt lets you choose a known card or the mystery box, Sleight of Hand is the same two-card choice with an open mystery box, and Serum Visions lets you keep both or neither of the two cards you see
Players can always just pay the requested mana, or, for each 1 in that cost, they can tap an untapped artifact or creature they control rather than paying the mana
The universal dilemma of the Infiltrator was that few decks that played it wanted to tap out to get him there, but sticking him on the board was so devastating that most opponents would consider themselves fortunate to spend that window of opportunity making him go away as trying to race him with a traditional three- or four-drop was pure folly and playing your own blocker, including your own Shadowmage or something a little bulkier risked a removal spell that would lead into a downward spiral of card disadvantage
While egregious play patterns was the primary reason for the bans, the card was also incredibly hard to find, and at one point during its time in Standard, was regarded as the card with the lowest supply, relative to the rest of the formats card pool
Feed the Swarm gives us enchantment hate in black, and if your opponent isnt using enchantments, you also have creature removal