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.