In the event a Raisin partner bank fails, your funds deposited with that institution remain insured up to the FDIC limit, respectively. This is typically up to $250,000 per depositor, per insured institution, per account ownership category. Keep in mind that all deposits you have at a bank — whether made through Raisin or otherwise — count toward this deposit insurance limit.
In the event of a failure, your funds would likely be temporarily frozen while the FDIC works toward resolving the failure, either through issuing payment to the depositor directly by check up to the insurance balance in each account (typically within a few days of the bank closure) or through the purchase of the failed bank by a healthy bank.
In the event a Raisin partner bank is sold, you would still have access to your funds; they would just be deposited at the assuming bank. You would be able to then deposit or withdraw your funds as needed.