It's almost feature-complete, tons of bugs have been eradicated already, and the stable release is should be 2-3 months away.
As for slow developments... I don't know where you're taking that? Godot has dozens of commits every day, and the pace at which 3.0 features were implemented is honestly impressive (I'm obviously biased as a Godot dev, but check the GitHub repo [0] and the monthly progress reports on the devblog [1]).
Godot has been in development since 2007, open sourced in 2014, and has had 400+ contributors [0] over 3 years vs 32 [2] for Atomic over 2 years. On the other hand, young Atomic is an overlay on the mature Urho3D project with 100+ contributors [3] and over 6 years of development.
Both Godot and Atomic have their strengths and weaknesses, and are IMO two great open source game engines - but as far as maturity goes, I think the sheer size of Godot (both in terms of community and git/issues/PRs activity) compared to Atomic says a lot.
[0] https://github.com/godotengine/godot [1] https://godotengine.org/devblog [2] https://github.com/AtomicGameEngine/AtomicGameEngine [3] https://github.com/urho3d/Urho3D