The good stuff:
- Unreal let's you easily alter the aesthetics of your project. The drag and drop material editor, the huge amounts of built-in high quality content let you easily reach AAA levels of quality, even for small prototypes.
- Blueprints are amazing for quickly prototyping concepts! Beware: any non-trivial math expressions will be a mess of wires, but thankfully there's a handy math expression node to help you avoid turning your code into a spaghetti mess.
- The community has a lot of experienced developers, which makes useful content and non-trivial tutorials easier to find.
- The architecture and the way everything is structured encourages newbie developers to write more robust code. Contrary to Unity, there's already a game state, game modes, an Actor-Controller model...
- Epic's experience has led to so much high quality content that you can easily pick up something like the FPS Tutorial and merely have to tweak a few values to suit your needs - We recreated the movement style of our first Unity game (Slinki) in a matter of minutes.
The bad stuff:
- The render pipeline is setup for realistic lighting, if you want something really stylistic illumination you can either "hack" your way around this with more complex materials or get down and dirty with the rendering pipeline.
- Don't get me wrong, Unity can be a bit of a resource hog sometimes. But Unreal seems to be an expert at making even our reasonable Gaming PCs suffer. Compile times will be measured in minutes for anything but the smallest projects.
- The way Unreal handles project files is simply not as robust as Unity's. Every time you move something, a file a redirector is created.
- You can't just drop a png file and use it as an asset, there's always going to be some sort of pipeline to get assets into your game.
- As a sidenote to the previous point, you will have to export your blender models as an fbx. Many things can and will go wrong with this process. I would write a tutorial but I found it changes a lot between versions of both programs and none of our solutions have been ideal so far.
- C# is gone. You will be using C++. This can be a curse or a blessing but overall C++ is not as great for quickly prototyping.
- It's not very stable. Seriously, it's not. The update to 4.9 broke transparent sprites on the UI. This is not a trivial and easily-ignored bug.
- The huge ingress of novice game developers has made communication with the Epic team and experienced members slightly harder due to a sea of (mostly) trivial questions.