I just downloaded and have been loving this. It loads pretty quickly, navigation is intuitive, and I'll finally stop forgetting that Nebula exists because it'll all be in my one big subscription feed.
Since I'm new to moving over to open source, I want to ask the veterans: is this as incredible as it seems right now, or is there something I'm missing?
The difference between "open source" and "FOSS" has already been discussed to death: https://www.gnu.org/philosophy/open-source-misses-the-point.en.html
Don't equate the two terms; they're not the same. You can argue that they should be the same, but unfortunately it's too late. Words are all made up and they mean what people decide they mean. In this case, "open source" means that the source is open, and nothing more.
Unreal engine is source available, and you don't see anyone calling it open source.