They let you play the attrition game a bit better but they do typically make you more vulnerable to aggressive decks, particularly red ones
Not just a new planeswalker card, but this dude has never been on a Magic: The Gathering card before in his life
Archmages Charm is extremely pushed, a solid permission spell, a good way to build card advantage for hitting land drops, and a fantastic tool against Deaths Shadow, Wild Nacatl, Goblin Guide, Pyxis of Pandemonium, Champion of the Parish, Aether Vial, Delver of Secrets, Hardened Scales, Amulet of Vigor, Benthic Biomancer, Glistener Elf, Noble Hierarch, Viscera Seer, and sometimes even Hexdrinker (just to mention a few)
I am sure that you know how cards that allow you to recycle resources from the discard pile are often pretty broken, right
The featured mechanic from The Ruinous Powers is cascade, an extremely powerful ability that gives you the option to reveal cards from the top of your library until you find something that costs less than the spell you cast, and you cast it for free