I think the one exception -- and this was an extreme example -- was the firmware of a medical device which could endanger human lives if modified. Things were locked down.
But generically? I'm batting maybe 75% with open source in highly-regulated industries.
You can certainly use open source in regulated industries -- the question is, can they risk the legal possibility of being forced to open source the rest of their stack if the a court determines AGPL requires it.
NDA / sensitive government work -> FOIA makes restrictions tough. I can request government source code.
Trade secrets -> The government can't really keep trade secrets. I guess if I were working with classified code?
Hard-coded exceptions -> Not specific to regulated industries but bad design.
Legal possibility -> This is not a legal possibility. A legal possibility is paying damages. It's the same if I break a proprietary license too (for example, accidentally ship out a copy of a library I paid for just one license with my product).
Above, I was referring to trade secrets in the private sector, but actually, that is one of the FOIA exemptions too.
Also, a lot of code running in production is poorly designed. I'd bet the vast majority of it is poorly designed.
Damages are one part of it, but you also have to return to compliance, which means you either have to follow the license or stop using the code. Given that the AGPL doesn't have linking exceptions, the concern is that might be all of an organization's code.
That's not to say that you can't use open source in industries where these might be concerns -- almost all other common open source licenses don't have this problem.
I've run into lots of libraries over my career with vague, conflicting, or otherwise questionable licenses. So far, 100% of the time I've been able to easily find an alternative open source project with a better, more clear, license. If both get the job done, I'll pick the one that doesn't keep me up at night every time.