The one that makes it trivial to carry over local patches is Gentoo. I carry a set of patches to every new computer and update them whenever neccesary. No overlay yet :P
This is the main issue NixOS solves for me. I don't have systemd units or cronjobs or random config files spread across the system. I have one config file/directory, which contains everything.
My beef with nix is the arcane syntax to get anything done. Yes, it's documented. No, it doesn't help that it reads like one of those strict, convoluted standards written by a committee of experts.
Also what causes problems for others may not be the same thing that causes problems for you. You could try accepting others' experiences.