Hello there gentlemen
Here's my honest input:
Depends on your requirements. In many aspects they are similar, they're both fully featured game engines with somewhat high-level scripting and heavily cross-platform. Here are the main differences in my opinion:
- Modern C#, highly performant Mono or coreCLR on supported platforms as opposed to Unity's stone age Mono, which includes a Garbage Collector so slow that brings anything heavy on processing to prohibitive performance levels.
- The (native) engine performance is also an order of magnitude better. I'll never understand why GameObjects are so heavy in Unity. In Atomic Nodes are as lightweight as it gets. Basically you have much more freedom to do things in a 'naive' way. Culled static nodes with no logic have almost no impact on processing.
- Some of the best in class opensource libraries. SDL2, Bullet, Box2D, Recast/Detour, nothings libs, assimp, and a kickass renderer which although a bit outdated is comparable to bgfx in performance.
- Full source code access with bindings generator. Need to draw a fully animated tilemap from data on managed side at the highest performance possible? No problem! Add a function to draw it directly in the C++ side with minimal data passing and it gets exposed on the managed side, so the heavy lifting has native performance. Found a bug and need to fix it immediately? No problem too. While Unity does license the source code, it's a big ball of mud while Atomic's elegant and simple.
- The editor. Atomic is progressing quickly but Unity's editor is ahead. It's fully scriptable and the API is mature and well documented.
- The asset store. Although a lot of assets have terrible quality, overall it's a good resource to get things done quickly.
- 3rd-party integrations. Ad networks, IAPs, social media, cloud services... they all have official ready-to-use plugins.
- Documentation/Tutorials. Unity docs are far from perfect, but there's no lack of tutorials, either official or not.
- API. Unity lacks some pretty obvious features like Runtime Navmesh, Decals and Input, but the features that are there are fully documented and the API is more battle-tested.
It's forked on Urho3D which is very mature and reliable.
If you have experience with game development, it's not hard to use, besides some unusual namings, it's pretty standard in terms of ECS and API.
I don't think you can just drop a .blend file in the assets folder use it like in Unity. You'll have to export your models. That being said apparently assimp does support blend files but I'm not sure if it's up to date or otherwise the current state of the importer, but I think supporting blend files directly is a plausible feature for the future, as well as PSDs.
Not sure what you mean. I'm not aware of any 'success' stories on Android so far. In terms of deployment Android can be a pain in the rear but besides some possible annoyances it's safe to assume in the worst case it's just annoying/tedious to deploy on it. Android isn't anything new at this point.
For 2D games, yes, Spriter and Tiled are two tools that could help tremendously. Sure, the documentation really needs work.
In terms of performance you'll hardly beat Atomic. Only a hand-optimized custom native engine stand a chance. The core is extremely simple and efficient, there's no obvious unnecessary overhead. I really believe Atomic is almost as efficient as it gets because it's extremely simple/lightweight and yet employs all the optimizations you'd expect in a top notch engine.