Its true that you lose the ability to regroup, but you still have the other benefits, and of course, a land based community is easier/cheaper than a sea based one. Also, regrouping wouldn’t be easy once seasteads get bigger. A land community can still “regroup” simply by having the constitutional right to secede. They won’t physicaly move away, but they can have political separation.
Regarding voting rights, they’re the same for immigrants, in the case of a referendum. Even if you have to wait 2 or 3 years to vote, that’s still more short term than the seasteads or Free State Projects, for instance.
as for being grabbed by a government, i think that would be far fetched, since the idea is a legal and democratic change of government. It would probabily still be part of the Commonwealth, and, if i’m not mistaken, the proposal was for the Falklanders to still count with protection from the UK military. And the UK allow some well know tax havens in their own possession, like Jersey, Guernsey, Isle of Man, some Caribbean territories, etc.
Someone taking over Falkland would be like taking over the Bahamas, or Liechtenstein.