When would you not use DCF in a Valuation

From M&I Valuation Question 3: "When would you not use a DCF in a valuation?"

The answer essentially starts with the idea that you shouldn't use DCF for companies with unstable cash flows. Intuitively, this seems to make sense as it might not make cash flow a good metric of valuation for a company if this is the case.

That being said, I am confused about the second part of the answer "or when debt and working capital serve a fundamentally different role. For example, banks and financial institutions do not re-invest debt and working capital is a huge part of their Balance Sheets - so you wouldn't use a DCF for such companies." What does this mean by a "fundamentally different role," what does it mean by banks not re-investing debt, and how can working capital be a "huge part of their balance sheets" when it is the difference between current assets/liabilities? Thank you in advance.