Ubuntu, like most mainstream distributions, has design choices and UI that abstracts away as much as possible the act of administering a Linux system. A lot of things you will do on Ubuntu are specific to Ubuntu systems, and even more so with distros like RHEL and Gentoo. Then you have oddities like GoboLinux.
It's one of the oldest sayings in the community that "if you know Slackware, you know Linux". This is somewhat less true for Arch but pretty close and still has the same spirit.
I've run Slackware, Debian, you name it and built my own LFS systems and run those as well. Slackware is as "raw Linux" as it gets, but due to frustrations with recent versions, I switched to Arch and haven't looked back for my Linux systems. It makes administration much simpler without having to do much that's specific to the distro and different from what you should expect in Linux.
That isn't something even remotely true about Ubuntu -- and apparently getting less so as time goes on.
tl;dr: what I'm trying to say here is that there are users and there are administrators. If you can't administrate your Linux system, you don't know Linux. Ubuntu tries to be a Linux system for users without needing to be a sysadmin. This is an idea that's "okay" I guess but in my opinion still not ready for primetime. Using a Ubuntu system isn't "knowing Linux".
