18.04 is missing basic functionality like the browser and store, and the script has dependency issues on 16.04. Am I missing something?
For now, Unity8 for the 18.04 (bionic) desktop is in early alpha, and nowhere near ready for daily use. Also, problems installing Unity8 on 16.04 (xenial) are known. See Unity8 Issue #64
Ubuntu touch on the Surface would be incredible!
Yep, like you, I'm eager to have Unity8 working on the desktop. My thinking is that many will want to try Unity8 on the desktop first, and if they like what they see on the desktop, they'll be more likely to install UT on a phone. That is, having Unity8 on the desktop could serve as a great way to market UT on phones.
However, for now, UBports is (rightfully) focusing on phones, including updating Unity8 to work with the latest version of Mir and adding full Wayland support. That work should be transferable to the x86_64 desktop. So my hope (as a non-developer bystander) is that, if things go well, a usable version of Unity8 will be installable on Ubuntu 20.04 LTS desktops sometime next year.