I recently worked on a small-ish desktop app that shuttled data between a HID USB device and a web service. We targeted Windows and OSX. The UI portion of the application was very simple and I still found the Xamarin/GtkSharp combination buggy and frustrating.
A TeamCity build system using NuGet worked fine for building both flavors, though the Mac build was really just a shell script that TeamCity launched. We used the Xamarin compiler for Mac.
In hindsight I wish we had taken (found?) the time to go all the way and write separate OSX specific UI code and not used GtkSharp. And not require the install of the Mono framework for Mac users-- that really kills the experience. Supposedly you can bundle in Mono with the Xamarin .Mac product but we haven't found the time to get that far yet.
*EDIT: I failed to mention explicitly this was C#/Mono. Also for those interested I used http://www.signal11.us/oss/hidapi/ as a wrapper around the Mac HID USB APIs and it worked fine.
This item: https://www.competehub.com/races/tri-for-the-y-sprint-2014 The data is all wacky: 1) The gmaps swim tab is in Germany, 2) The Logistics > Start location is on Long Island.
(I happen to live right near that race)