We maintain internal forks of Eto and Nancy. They are easy enough to grok for us to modify and maintain, even when development upstream dies.
Most of the user interaction is happening via the web app served by the server. The local clients are "just" for configuration, comparable in complexity with e.g. Tunnelbear or Glasswire. We use some custom grid elements, which have been easy to implement.
Eto is the only framework out there which gives you native GUIs everywhere. That's what was most important to us. Native speed, native OS integration (tray menus, notifications etc.). It's also really easy to learn for devs who might have been WPF-exclusive before.
A sort of follow-up question: have you contributed things back to Eto? Were they responsive?
Thank you for your detailed answer, by the way! :)
No, most of the small customizations are very specific to our application. Mostly how closing events or notifications are handled. TBH at least I don't think much about Eto day to day, it's just there and works. Questions mostly have been answered by existing issues, where the author seems responsive enough.
Also, do your GUI clients have fancier UIs? For example, have you implemented your own grid view, something where you put custom controls in the cells, images, graphics, etc.?