Sure, your opponent might have ten beasts in their green deck, but Callous Oppressor would still be able to steal their four goblins, three elves, and two other creatures that weren't beasts
They take advantage of the Initiative mechanic to gain both card and board advantage, and use that advantage to play lock pieces similar to Shops decks or powerful creatures early enough to cause problems for their opponents
The card economy of these kinds of spells are normally traded off with losing access to the remaining cards, aside from an occasional shuffle, but the power level rises when suddenly the cards being placed there are going to be our next draw
You aren't going to win against the Friday Night Magic sharks with these and I wouldn't recommend them for those looking to play seriously (whatever that means) but for kitchen table Magic, there is tons of fun to be discovered with them
Suddenly everyone else loses all their cards in hand, their lands, and their creatures
To check if your card is part of the base set, look at the bottom right corner of the picture box