The U.S. Government forced the Cayman Islands to expose people using the Island Nation as a tax haven, thus negating the benefits of the islands and large parts of the clientele have gone elsewhere. Since these are only expected to be a half mile offshore they are going to end up being subject to whatever nation-state they are offshore of. Were they to try this in the deep ocean I suspect rogue waves would be an issue. Either way this isn't likely to succeed politically, although I see no reason why technologically it wouldn't. It's more likely going to end up being a playground for the ultra wealthy to sometimes visit and say they have property on.
It could end up being a wealthy playground, but the people doing this stuff would consider it an utter failure if that's all they accomplish. What they really want to do is sequester billions of tons of carbon, provide billions of tons of food, and provide an economic opportunity for a billion of the world's poor.
I think whether that's possible depends mostly on how cheaply these things can be built at scale.
