Ranked in these QuestionsQuestion Ranking
Pro Lots of resources to learn from
Unity3D provides an exhaustive documentation where everything is given a full description supplied by a number of examples as well as video and text tutorials and live training sessions to understand the ins and outs of the engine. In addition there's an ever-growing community that can offer advice to help resolve any situations that may arise.
Along with the official Unity resources, there are many high quality (and often free) third party tutorials available.
Pro Allows for rapid prototyping
Unity's modular system and usability allows for quickly developing a prototype of an idea. It has features like drag & drop editing, shaders, animation and other systems already in place to allow diving right into developing a game.
Pro Lots of assets can be found in the Asset Store
For those developers who can't afford an artist, or aren't skilled enough to create their own art, Unity features an Asset Store full of a wide variety of free and paid assets that can be easily added to a game. The Asset Store has more than just music and art. It also has code and modules that can be added to games including unique lighting or GUI systems. It also has powerful asset management and attribute inspection.
Pro Easy learning curve
The way the editor is structured, by setting scripts on objects, and the use of a high-level language, C#, makes it easy to learn.
Pro Great editor
The editor GUI is very powerful and intuitive. It allows pausing gameplay and manipulating the scene at any time as well as progress gameplay frame by frame. It also has powerful asset management and attribute inspection.
Pro Very popular
Unity is a proven game engine. It is used by a wide range of developers - from small indies to triple-A companies such as Microsoft, Paradox, Square Enix and Sega.
Pro Free for mobile development
Unity allows free mobile development. There are some limitations.
Pro Can be used for free
As long as the development company makes $100k or less, it can use the free version of Unity to release games.
Pro Over 20 platforms
Unity offers over 20 platforms for publishing including mobile, console, web, VR, and more.
Pro Works with 3rd party IDEs
Pro Has a great animation system
Unity provides a great state machine animation system called Mechanim allowing to separate animation from the model and assign the same animoations to different models.
Pro Royalty-free, one-time licensing fee
Pro Has awesome plugins
Pro Very optimized
Unity runs very smoothly even on systems that are considered "weak" by today's standards.
Pro Offers choice of scripting languages
Pro Powerful standard shaders
The built in standard shader in Unity 5 is incredibly optimized and supports PBS/PBR.
Pro Can create custom forms and tools
Pro Flexibility is provided by a strong component programming model
Pro OUYA support
Con Hard to maintain projects due to vendor lock
Unity3D is proprietary, closed source game engine. Unity asks money for features like basic version control support, etc. It is impossible to migrate a game from Unity3D in case performance does not satisfy growing requirements of a project.
Con Asset bundles can be cumbersome
Asset bundles are a way to load external resources that are not packed with the game or application and offered as a separate, optional package. However, they may not be compatible between versions or even platforms (you have to create them separately).
Asset bundles need to be loaded and unloaded, avoid concurrent loads from web or cache or a naming collision can happen. You can find workarounds with static objects (load
obj files and textures by code), but for animated game objects you are pretty much stuck with this.
Con Garbage collection can't be turned off
Given the use of C#, the memory control is out of the developers control, this can be good, but not controlling memory means that the garbage collector can trigger at any time and ruin performance.
Con Weak memory management
.Net libraries are slowing it down, memory safety is compromised, classes have to be implemented to manage objects in memory, like object pooling.
Con Very self-centered engine
Unity3D uses very unique approach for doing things. Most of the knowledge acquired while using it, would be completely non transferable to other engines. Advanced Unity3D programming is really dealing with Unity3D bugs, and finding loopholes around engine issues - nothing to do with graphics, etc. Skills which would be valuable with other engines.
Con Very bad terrain
Native terrain creates a lot of draw calls which is bad for performance.
Con Encourages bad coding practices
A lot of Unity code feels like a hacked blur of arguable coding practices. C# and .Net usage in Unity is questionable. A lot of the API is done in "C Style" (public static methods, available at all times), encouraging the use of public fields for everything, a lot of questionable implicit casting. The list goes on.
Con Increasing number of bugs
With each new version things may stop working ,for example with Android, some bugs are never getting fixed, like the freeze bug with adb.
Con Adds too many features without fixing earlier issues, rapidly increasing number of bugs that will never get fixed
Unity continues to add many new features without fixing earlier issues. Unity is either understaffed, overambitious, or both, resulting in a continual increase of problems and degraded experience across a number of platforms. Many bugs are reported daily and never get addressed, and there are many bugs from previous versions that are never looked at or fixed.
Con Bad batching support
Draw Call Batching is done automatically and does not include Skinned Renderers (eg. characters). Also for dynamic batching, meshes need to have less than 900 vertices.
Con Limited 3D modeling capabilities
While Unity itself lacks proper 3D modeling capabilities, tools such as ProBuilder or MXD are available in the asset store that add that functionality within Unity.
Con Mobile builds (Android, iOS) take about 18MB at least
Even a Blank Project, Needs 18MB for the APK file (on Android).
Con Price is very high
Con New il2cpp script backend is show stopper to some iOS projects
Il2cpp script backend is required to build arm64 app, but it's still very buggy and not production ready.
Con Variety of scripting languages complicates compilation order
Con Boo and UnityScript don't have many learning resources available
Con Comparatively high learning curve
Although C#, JS, and Boo have documentation available online, it can still be difficult to understand the library and Unity's component based system.