Inevitable fights/headaches when one person wants out -- yes, you planned for it, but it's still a hassle.
All the pains of homeownership (leaky roofs, pipes, HVAC problems, etc.) x 1000 due to difficulty of getting a construction crew to the island.
Lack of a police force to enforce basic property rights against squatters/hunters/other "intruders" with different ideas about the meaning of "private propety" than you
Water: is this all being brought to the island, or do you have a plan for on-island desalination? Are you prepared to regularly service the desalination plant and/or haul giant jugs of water to the island constantly?
Medical care: when someone breaks a leg, steps on a sharp stick, etc., where is the nearest hospital?
This article reminds me why we live in a market economy, because tastes and preferences differ, and there's no way I'd buy this, but great if you guys want to.