The poorly funded ones make the news, but there are many healthy ones that don't. It isn't agency risk that causes problems, it's a combination of increased longevity and low interest rates.
A big problem with 401Ks is predatory investment advice. "Advisors" channel money into high-fee funds and commissionable investments and often "churn" money to generate additional commissions. The Obama administration cracked down by instituting a "Fiduciary" standard for retirement funds, but Trump has delayed implementation of the rule twice.
401ks aren't perfect, but it's better than the above system. There's a reason no one sells annuities even close to what the government offers in it's pension benefits. It's ridiculously costly without using funny numbers.
Agency risk is always there when you hand over responsibility of something to someone, and it's especially acute with big pots of money. The best investment for retirement is to raise children who will raise grandchildren who all look after each other.