Public Defined benefit plans will mostly all collapse. Blame all the cheaters over the past 25 years who managed to get 8 hours of overtime per day for their last 3 years and now at 50 years old are retired in Florida making 80,000 a year with full medical.
State and local plans have different problems from federal plans. State plans suffer from the ability to spike benefits by working overtime, too high of a payout, early retirement ages, and depending on investments to make the plans solvent. Add in some plans where the governments have skipped payments and you end up in big trouble.
Federal plans, which were reformed back in the 80's, have lower payouts of higher salaries. The most obvious issue is that unlike most state and local plans, the federal government as a whole operates at a deficit. For all of the torch and pitchfork types, you will be in happy to know that one of the largest holders of federal debt is the federal employee thrift savings plan, a 401k type investment plan. When the government defaults, those employees will be stuck holding the (empty) bag.