After buying out HSBC, Capital One is likely to be the sole significant subprime credit card issuer in the US. It is also a leading subprime auto lender. But it does not have a sustained, substantial track record in any major businesses except card and auto.
Not in consumer deposits. Not in services to any market except consumers. Previously it tried and then gave up on some other lines of business, such as mobile phones.
Why worry? One concern might be that at least in recent decades, there have been no large scale, long-term successes in American subprime lending. Household International collapsed a few years after its purchase by HSBC. The mobile home lenders are all gone, as are the subprime mortgage companies and the finance companies. CIT went under.
See a full detailed article on this subject in American Banker