The cost of deferred compensation can be wildly understated for decades (and continues to be) by using erroneous investment return and mortality rate assumptions. Also, the risk of corruption remains for decades as you have a huge pot of money which many are fighting to get a piece of with little transparency.
No one actually considers the figures put out by IL/NJ/CT/Chicago and other distressed governments as accurate, and if they had to abide by the rules that non taxpayer funded entities have to, their debt would easily be 30%+ higher than stated (based on an assumption of a 1% change in discount rate moving liabilities ~15%).
I agree that politicians can also hide debt by not properly accounting for infrastructure and long term maintenance costs, and that is also a big problem.