Hey Tristan,
There are a few reasons why the UI was updated and for now a causality of that is running it pre-Maya 2017. For one, the UI code and zBuilder code were completely separate while still overlapping functionality in a lot of areas. With the new one it is built on top of zBuilder meaning that a lot of the logic didn't need to be duplicated. We could remove plenty of code and it got to be problematic to keep them in sync. This means though if its implemented in zBuilder the UI will have access to it.
In Maya 2017+ the viewport changed in such a way so we cannot select Ziva items by clicking on them. The UI does make that simpler.
I am just going through some of the thought process.
If you want to get the old one back it isn't too hard, though the code for that is no longer shipped with the plugin. It can be downloaded from here:
https://bitbucket.org/zivadynamics/ziva-vfx-utils/get/1.0.4.zip
That is the previous release of ziva-vfx-utils.
Download that and unzip it to a temp area.
Identify where your ziva-vfx-utils are located. If you installed this on windows with the installer it will be someplace like this:
C:\Program Files\Ziva\VFX\Ziva-VFX-Maya-Module\ZivaVFX-Maya-1_4\scripts\
replace the zUI directory in the current path with the zUI from the download. That should get you up and running with previous UI. The command to launch it is the same so it will work with the new menu item.
Let me know if you have issues with this.