I read that OpenSuse ported Ubuntu's Unity desktop and made a repository, also available in Fedora and Arch. I was wondering if, besides Ubuntu, any other distro (or spin) is using Unity as default.
Well you just answered yourself. Yes OpenSuse, Fedora and Arch have a repo for Unity. In the Ubuntu ecosystem, all direct Ubuntu based distros (Lubuntu, Xubuntu, Kubuntu...) have Unity in the package manager, so you can install it if you want to change from the default this distros carry.
Outside of the Ubuntu world (In other distros not based on Ubuntu directly) we can see the following:
Arch Linux offers Unity and Unity 2D for low end systems.
Fedora and OpenSuse do offer Unity, but this need to be installed from a repository.
Other distros like Frugalware and Leeenux have or had Unity in some version or another and at some point in their development.
The small amount of distros offering Unity as an alternative is because:
Unity is/was still under heavy development so the decision was on hold.
Unity uses Compiz which some distros do not want
Unity has compatibility problems with Gnome libraries
Unity is a radical change.
Of course, there are many benefits of using Unity, and most of this points in later versions were fixed, but depending on the distro (Is it orientes towards and old linux user, is it for low end systems...) they would use or not use Unity.
I myself, can't really go back to another since Unity really covers everything I do in a strange way that after using it for a week had/needed to stay with it.