Depends on what you want. If you want to achieve the same results that are possible with Unity, you would need to use some more complex opensource engines like

Depends on what you want. If you want to achieve the same results that are possible with Unity, you would need to use some more complex opensource engines like Turbulenz and would be restricted to WebGL for now. Rolling your own engine doesn't make sense other than learning and maybe for simple 2D stuff. If you want to create games in a very efficient way and have a quick time to market supporting a lot of different platforms, there is hardly a way around Unity now.