I disagree that any retroactive effect of reneging would be "highly unusual." The circumstances that would lead Congress to renege on Roth IRAs--such as the perceived need for tax revenue--would also most likely lead it to renege retroactively so as to mitigate tax revenue losses.
If Congress were to renege on Roth accounts, however, the inclusion of the income tax in the taxable estate that occurs with traditional accounts would also occur with Roth accounts.
This article considers an additional source of uncertainty: the possibility that Congress will renege on the tax-free treatment of withdrawals from Roth IRAs.
Although the probability that Congress actually will renege on Roth accounts may be impossible to ascertain, the analysis here can help one to identify those circumstances where it may influence the decision between Roth and traditional accounts and those circumstances where it may not.