The bond insurers are rated by Moody's and S&P the same way which bonds are. I think if there were serious potential issues on the horizon, the rating agencies would (hopefully) be out in front of it downgrading them, and that hasn't happened with the majority.
At the end of the day, my view is that we know munis are about as safe as you can get next to treasuries. We have 50 years of historic data showing it, we also have a fair number of indicators of which bonds are more likely to have issues. Even so, defaults are extremely rare, and even when there is a default, it's generally cured, and generally in full. How many defaults were there in 2008-2009 of rated issues? Even with Meredith Whitney calling for total collapse?
I believe only one bond insurer is suspect - National Public Finance (part of MBIA). I say that because they defaulted on fully covering Puerto Rico insured bonds they issued insurance on. They paid some portion, but it was significantly less than what they were fully on the hook for.
https://www.reuters.com/article/bri...l-public-finance-guarantee-corp-idUSFWN1PC1BY
I also have some Illinois bonds - not at the state level, but specific cities/counties/municipalities. Some insured, a few pre-refunded issues. Nothing significant.