The trouble is how they compose with the expectations of code you don't write. It's just not normal in C to expect execution of your function to abort halfway through, and so memory leaks and broken state are almost guaranteed when they're mixed with the wrong third party library.

It might work great when you wrote it, but it might not even survive the next change to the codebase, and 10 years out, who is to know what programmers will come along and plug new stuff in without noticing the stack magic, etc

edit: another aspect of the same issue is that it can be difficult or impossible to write generic longjmp-safe code for many kinds of tasks. In C++ you have stack unwinding to e.g. delete large temporary heap allocations during a complex operation, but no such facility exists in C.