A Unity ID allows you to buy and/or subscribe to Unity products and services, shop in the Asset Store and participate in the Unity community.
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Like mentioned before Unity does not lock you in to any methodology. I've worked on mid-size projects ( 6+ programmers ) with an MVC model....
This is not how Unity3D is structured. In theory your coders could build all game systems decoupled from mono behaviors / GameObjects etc.. But...
Here's what I've run in to: Middleware Unity is sorely lacking in some game engine features; cross platform input / controller support, UI...
Agreed, this is essentially a regression for any game upgrading from 4.x to 5.x. For now it seems my only option is to rethink my scene lighting...