Munis are “safe” in the near term because very few general obligation bonds are at risk of near-term payment default, because of inability to make interest payments, or to refinance maturities.
A few years out, it gets worse: as the “hump” of retirees approaches, the munipal bond market will start to demand higher interest rates for new long-dated municipals, and might eventually refuse to buy them at any price. This will drive interest expenses higher and weighted average maturities shorter, and eventually some municipalities will hae payment defaults, and many others will face terrible choices in terms of budget cuts and tax increases.