This device uses a third-party firmware called MrChromebox, featuring the rabbit logo on startup. While it has been widely used to convert Chromebooks into regular computers and has been trusted by enthusiasts, I cannot guarantee this firmware is safe to use. By purchasing this device, you accept the risk that comes with running a modified device.
There are 4.5 GB of space remaining after a fully featured install, but there is also a working MicroSD slot. I tested the HDMI on a previous Linux install and it worked, along with HDMI audio, so chances are this works on Ubuntu too.
Some advantages of installing a free OS like this are to keep your unsupported computer out of the E-waste, and to make it so you no longer need a Google account or an internet connection to use your Chromebook.