i use this at work, and its great. Only downside is, that the buttons are hard to identify and move depending on the size of the screenshot, so you always have to search for the function you need.
For the longest time I used gnome-screenshot via shortcuts that execute terminal commands. Nowadays, I use my dedicated print screen button (that probably just executes similar commands and uses gnome-screenshot on the background).
On Mac, with multiple displays, it seems to be broken. Sometimes it targets the wrong display, and mixes the resolutions. Have tried for a while because it seems perfect on Linux.
For Windows, I recommend ShareX. It feels more modern than Greenshot, has more features and is more customizable. And it's open-source: https://github.com/ShareX/ShareX
Same page has instructions for choosing which key-combo should bring up the GUI. I'm on Ubuntu and had to do special steps to make it work with the PrintScr key but it's so easy to use now!
Flameshot was great, but for me, no window select (on Win) and no plan to implement is a deal breaker, thoughi understandthe reasons.
Also breaks when moving between docks - has to be restarted, and pinned images go under the screen, if pinned at the bottom (they always shift down).
While I like the UI, it makes it awkward to quickly find the icon needed on small crops, since they keep shifting around.
.NET works on non-Windows OSes too, at least enough to have a GUI. Avalonia is cross-platform for example. Not only across desktop but also mobile and web.