More specifically, issues for one distro (or maybe family of distros if applicable) and not other distros.
I’ll start: NixOS. I love how I can have my whole operating system configuration defined deterministically with configuration files, but what I believe is a serious flaw, is when you choose the “unstable” channel, the channel for receiving the latest packages rather than ones up to 6 months old: stable. Unstable package often build dependencies on device, and I’ve often faced build failures, why are new package versions given if they fail to build?!
Why can’t Nixpkgs backend ensure that new package versions build successfully before shipping them to end users?! It would take a lot of work to do so, but I can’t think of any other distros that have this problem: where installing a new package version that came out a few days or even a week ago, has a chance of failing after its made available in the distro’s official package manager.
There are a handful of workarounds for your NixOS configuration in this case, but it happens too often for me and I shouldn’t have to edit my config for to work around it.
It’s the kind of work that can be automated. Most of the Gentoo tinderbox build-everything-in-the-tree-and-file-bugs-on-failure projects (there have been several over the years) have been run by a single dev. I’d assume that Nix could do the same.
Issues with Gentoo? The only one I’ve encountered that I didn’t sign up for is the occasional difficulty finding documentation for some obscure and unpopular option, like downgrading the freetype interpreter to make it render pixel-aligned fonts the way I want it to, or bridging networks in qemu to allow a VM to address other machines on the LAN without systemd or NetworkManager (still trying to figure that one out). With Gentoo, unlike some other distros, you really need documentation—it isn’t going to enumerate the possibilities in a dropdown.