It simply moves the investment work and risks to the individual. It's a minefield for the individual and vastly more complex to manage (individuals are effectively forced into being investment managers... like that's an easy job?).
It's ultimately just one part of the seismic shift in wealth to the 0.1%. "Sustainability" is just the thinly veiled excuse.
The employee already caries investment risk, although they have no control over the investments. If the employer chooses poor investments for their pension fund, it can lead to the pension being terminated. If the plan is insured by the PBGC, then some benefits are guaranteed, but the PBGC itself is underfunded: as of the 2015 annual report, it has $164 B in liabilities and $88 B in assets; if your employer can't pay its pension obligations, and teh PBGC can't either, that's going to be pretty complex too.
If the plan has any decent target date funds, it takes about five minutes to guesstimate your retirement age and pick that. It's probably not the best choice, but it's a reasonable one.
If market returns don't meet or exceed expectations, you end up broke on the street (and likely dead). (EDIT: Market returns have been, over the last decade or so, lower than expected; there is a school of thought that returns will no longer be as high as they were historically). [1]
That's the beauty of Social Security: it's guaranteed not by market forces, but by the existence of government. So long as the US government exists and has the ability to tax, those who rely on SS and Medicare will be provided for.
It's ultimately just one part of the seismic shift in wealth to the 0.1%. "Sustainability" is just the thinly veiled excuse.