Zorin OS Upgrade Tool

Aside from the obvious differences between 16 and 17, including the changes listed here:

https://blog.zorin.com/2023/12/20/zorin-os-17-has-arrived/

and here:

https://blog.zorin.com/2024/03/07/zorin-os-17.1-is-released/

Which user customizations, if any, does the upgrade tool not maintain post-upgrade exactly, or nearly identically, as they were pre-upgrade? Anything that stands out that devs are aware of or that users have come across during their upgrade attempts?

Not sure how to answer that question. For the Core/Pro, GNOME builds the desktop and Zorin adds some extensions to help modify the look and/or functionality of Zorin overall.

On Xfce, I they tweak it to their liking giving it the Zorin look/feel.

You can test it out before using the upgrade tool by downloading the live USB and test it out on your machine and evaluate your particular areas of concern.

The Zorin 17 series, defaults to the packages in Ubuntu 22.04 LTS but many popular packages (like LibreOffice for example) are maintained in Zorin's own repositories. This allows some newer applications to be backported, making Zorin a very robust distribution.