I have seen other posts saying they were working in Godot engine. As long as you are not using pre-existing assets or code, and you have a game at the end of the jam, you will be fine.
What about the prizes? If you're working in Godot or something else, and you happen to win, you get a Unity Pro licenses... That's sounds like a bit of a waste and mild slap in the face, especially if you chose a FOSS engine because it was FOSS.
We are giving away Unity prizes because these communities are mainly using that engine, but we allow any engines / frameworks to be used in the jam. If you are using a non-Unity engine and you win something it's up to you what you're going to do with it (whether you're just gonna throw it away or give it away to someone who does use Unity).
Also, I think the bigger reward for winning is that you won, your game out of hundreds was chosen as one of the best ones, but just my opinion :D
