The P/S ratio is meaningful even for distressed firms, since sales revenue is always positive. This is not the case for the P/E and P/BV ratios, which can be negative.
In the P/BV ratio book value is an appropriate measure of net asset value for firms that primarily hold liquid assets.
Analysts use several different definitions of cash flow (CFO, adjusted CFO, FCFE, EBITDA, etc.) to calculate P/CF ratios.
When earnings are negative, the P/E ratio is meaningless.