Well, mon ami, I have ended up on a slippery runway (31R at JFK) in a DC-10 with the cockpit over the grass but the wheels, luckily, still on the concrete.
With my hand on my heart I could not absolutely state that the reversers saved the day although I still had full reverse applied until turning off the runway despite the usual convention of being out of reverse at 60 knots.
The fact is that reversers are only of any effect at high speeds and, although they make us feel better with all the noise and vibration, I'm not that sure that they are all that effective.
The brakes are much better.