A lot of people are flying with systems designed for aviation but made 20, 40, even 60 years ago. And besides mechanical failures, other things can go wrong - see some anecdotes posted above and below, such as kinks in hoses, hoses pulled off of a fitting, etc. No matter how official and expensive a system is, I would use a flow meter to verify the flow (and still need to check that hose still reaches the mask etc). An exception is an EDS system, for which a flow meter is useless. But at least you can feel the puffs of gas in your nose (if using a cannula), and the EDS at least has built-in alarms for some sorts of faults.