It's a colossal pain in the ass to get working.

They recommend using Ubuntu. And they are not joking. Just click on the other distros to see the amount of hoops you have to jump through. You can't even get overlay2, which offers the best filesystem performance. Not to mention the benchmarks I've seen have slirp4netns at about ~3% of the performance of root veth. I would consider both of those incredibly meaningful limitations. You have to use sysctl/setcap to get ping working, to bind to ports <1024, muck with systemd to get user processes starting at boot, etc. etc.

It's a tough sell when you can just install root Docker with a single package command and never have to worry about a bunch of caveats that might just break or change on the next release.

podman's rootless suffers from the same performance issues as docker's. This is all the same exact tech.

Rootful-<tool> is always going to be smoother to get working. Docker can be a lot smoother than it is today, though.

It's not that big a deal doing it from scratch, but most users of Docker were already using it and have it setup to run as root. Undoing that is not trivial, definitely not as simple as just uninstalling and reinstalling rootless. You have to nuke your existing containers, images, and volumes, and depending on the package manager, it's not always obvious if uninstalling Docker will undo all the networking and user configuration changes it makes. Just out of curiosity, I uninstalled it on Arch right now and it didn't get rid of the docker0 virtual bridge device, it didn't remove the iptables rules, /var/lib/docker is still there with everything in it. The docker socket is still there, but I'm guessing that would go away if I rebooted, assuming the systemd unit creates that on login.

Extra packages/steps/incantations that to be performed/installed, introduction of a docker context mechanism that requires understanding, lingering inconsistencies with external filesystem mounting as root owner, incompatibility with most existing docker scripts and compose scripts, confusion on how to get back to root mode, etc.

It just didn't feel like a turn key solution. I have no idea how this would work in CI/CD systems though docker doesn't always need to be as secure there. Docker is a great tool but, like a lot of tech, in the mad rush to market, security was an afterthought and nowhere has it felt more clearly to me than in rootless mode.