Nobody's raided the Cayman Islands yet. I don't know whether seasteads would end up being offshore financial centers, but plenty of island nations are providing all sorts of financial services to criminal organizations without significant consequences.
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.
The Cayman Islands are a British Overseas Territory. A seastead isn't going to be an overseas territory of a significant power in its own right that is also a NATO member and permanent UNSC seat holder.
True, but there are other offshore financial centers doing the same thing, which are not under the British umbrella, including several small European nations, several in the U.S. sphere of influence, Asian centers such as Singapore, and independent oddballs like Uruguay and Somalia. None of them have faced serious consequences of any sort.