This link (edit: I link to https://github.com/NixOS/nixpkgs/pull/522245#issuecomment-45... but probably the anchor is stripped automatically?) is the clearest that I can find to point out what's the problem in a neutral tone with links to issues you can track yourself.
There is other less civic discussions around the issue that I decide not to cite.
One comment describe the situation aptly: 3.4.1 contains security bugs, later versions fixes it but contains regressions. So choose wisely.
*Apparently* both are AI related: the security bugs are probably discovered by AI. Rushing to fix lots of these results in the use of AI, which leads to regressions.