I did encounter a show-stopper bug in elm/core in my app in Jan. 2019. It was fixed in Nov. 2019.
After waiting about a month for my PR to be merged, I began applying the fix in my nix build script. When the fix was eventually merged, I updated and stopped patching.
Should I encounter any other show-stoppers in elm/* code in the future, I plan to use a similar approach. The main downside is that the fix didn’t kick in during development, only in “real” builds. (In this case, that was fine, I could just avoid the known landmine of albums containing more than 100 photos during development.)