With a state pension, a government can exclude people from reaping what they have put in, retroactively. I don't expect to be able to claim a UK state pension -- they can just keep putting up the minimum age from 65, to 66 to 70 to 75 to 80... until the scheme is removed entirely.

Whereas with private pensions the values go up and down but are unlikely to hit 0 and stay there -- unless it's really mismanaged!