Mill players rejoice! Fractured Sanity is the most potent focused mill card we've ever seen!
If we're just looking at raw numbers, we could draw comparisons to Archive Trap or Brain Freeze (thought the latter is not legal in Modern). Sanity Grinding can potentially hit a higher number of cards, but requires deckbuilding restrictions and luck (though Fractured Sanity itself definitely contributes towards it!). Glimpse the Unthinkable is perhaps the gold standard for this type of card, and Fractured Sanity provides a similar rate.
But the real added power of Fractured Sanity comes from its ability to cycle and still provide value. Dedicated mill decks are kind of like a blue version of burn – the effect you're going for is super difficult for your opponent to interact with, but your gameplan has to reach a critical mass of the same kind of card in order to close out the game. Fractured Sanity helps greatly with just being able to smooth out your draw while still contributing to your gameplan, in the times where you just need to curve out properly or find land drops. When you don't, it just straight up mills a cool 26% of their starting library!