We’ve met 8 advanced conqueror’s Haki users out of the, like, 20 known conqueror’s Haki possessors. So AdCoC pretty much has a 40% learn rate.
Conversely, we’ve met 3 future sight users, 3, out of the literal millions, billions, or possibly even trillions of people who possess observation Haki. So future sight has about a 0.000000…..0001% learn rate.
You tell me which seems harder to learn.