Indeed, this is terrific for a Roth, all that 'ordinary income' dividend ends up totally tax-free. Inside a regular IRA, not so, all those accumulated dividends will end up fully taxable upon withdrawal. That's where I have AGNC and also some CIM, considering other high-rate stuff like NLY.
As to the fellow who doesn't trust the gummint to forever keep ROTH distributions non-taxable, I too share your fear and skepticism of what they may do, indeed at one time all SS income was non-taxable, not true now, and the future?
You NEVER have a contract with the gummint concerning tax issues, rarely are you ever 'grandfathered'.
But, it's unlikely to be treated as bad as a regular IRA anyway, so I don't see any heightened risk keeping it in a Roth vs. regular.
As to the comment that the income tax on SS goes into the SS fund, I find that hard to believe because there is nothing specific on the 1040 that breaks down the amount of tax you paid on just the SS income, and would that be calculated at the bottom, average, or marginal rate of your tax? I was under the impression that it's just more tax collections that go into the general fund.