Creatures (19) Planeswalkers (4) Lands (24) Spells (13) Sideboard The combination of unbelievably flexible and efficient discard and removal, combined with Dark Confidant to fuel the whole thing, has been a defining strategy in Modern on many occasions
The biggest downside to this card is the cost, when can set you back upwards of $240 for a copy of this card.
While it's an expensive addition, it's worth the cost in any mill deck to throw in a copy of Bruvac, the Grandiloquent
This is why the leaves the battlefield trigger really helps because it keeps your counters in play like modular, but also counts if it is bounced, flickered, or airbent
Popular ones include planeswalker strategies since they deal with loyalty counters, and poison, since proliferate can tick up a players poison counter count
Once again, this is their color identity, which massively restricts deckbuilding options