After working with Unity Editor I have realized that it has several drawbacks from my point of view.

• Build time for HTML5 was extremely long even when the game was very small. It took several minutes to build the application.
• Personal license always displays logo of Unity and I had to wait for the logo to fade out even when doing small iterations and testing of the game.
• Unity is close source and even when I found the bug, Unity QA was not able to simulate it and they dropped the issue.

I started to seek for other engines and I was surprised by myriad of engines described at Wikipedia. The list is quite exhaustive and it is little bit hard to recognize the maturity of each project.

There is another list at GitHub which contains just open source engines. Projects are sorted by stars which makes it easier to see how is the project popular.