Let me begin with saying how many GNU/Linux distributions are out there on the web, and how we are spoilt for choice when it comes to picking one that meets our needs. Over the course of many years, small niche distro’s have appeared which cater to the needs of their respective user bases, but then there are plenty of users (the vast majority) who want a mainstream distribution which is easiest to use. Ubuntu ticks that box for many people, however there are some issues which may warrant moving to Arch Linux.
Ubuntu has a good selection of packages, but the problem resides in the frequency at which they are updated: not often enough in many cases. This can be remedied by installing third party repositories to get cutting edge versions of your favourite packages. Another problem with Ubuntu is that it has, pre-installed, a wide selection of packages thereby contributing to the startup time; if you're an experienced Linux user or developer, it's worth considering the option of choosing Arch Linux, and installing things piece by piece so that your system starts up and shuts down quickly amongst other benefits.
I've installed Arch Linux myself and since doing that, my system performed quickly, startup and shutdown times decreased and I benefitted from the newer features and bug fixes provided by the latest versions of software. However, installing Arch Linux isn't easy, as it's the user's reponsibility to perform the installation tasks themselves, step by step, however the folks who maintain the documentation for Arch have kindly put together a comprehensive guide for a standard installation process here: https://wiki.archlinux.org/index.php/Installation_guide. Follow that and you should be fine. The only problem I encountered in installation was connecting to the internet, as you need to run the DHCP (Dynamic Host Configuration Protocol) service, dhcpd, to assign an IP address for your machine.
The main problems I've had with Arch is with compiling kernel modules, especially those that come with Oracle VirtualBox, and also the requirement of the user to interact with system services, such as dhcpd (as mentioned previously). Things that just work on Ubuntu may require further configuration on Arch to work as well.
In the end most people will choose Ubuntu or one of it's other flavors over Arch Linux because it’s easier to install, use and configure, but if you're willing to spend a bit more time installing Linux, and also just want a minimal set of packages installed, Arch Linux is for you.