It is rather like the Primus in its role, it buys you enough tempo and board presence to be able to easily take the game from that point with your other cards
P/T Number 309 Language English Legal Formats Commander, Historic, Legacy, Modern, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Pauper, Pioneer, Standard, Timeless Rulings (3/19/2021) If for some reason you can't win the game (because your opponent has cast Angel's Grace this turn, for example), you won't lose for having tried to draw a card from a library with no cards in it
While Gen 7 does have its fanbase, it seems unlikely that it will be the target of this huge set
Typically the kinds of deck that want coverall counterspells also want to leave mana up to be reactive, having to tap five mana in your upkeep will make the turns following a Pact of Negation deeply uncomfortable until you have lots and lots of mana