> Since my biggest annoyance with proprietary software or in this case game engine is that you can create the most elaborate sub-engine (for example a fully-dynamic RPG) and see it all come crashing down because some dimwit at Epic Games reeeeally wants to push the latest eye candy that breaks your sub-engine.

Uf, feel that one. If the engine isn't designed with your genre in mind you may be making some significant structural changes(which will be interesting to see how open source engines handle that).

Even if they don't take the engine a different direction the up-leveling cost of getting new features/bug fixes is significant. Back when I was working in industry we had one person who's almost fulltime job was to merge -> fix compile errors -> fix runtime issues -> checkin change and uncover more edge cases that got broken -> repeat process, it was brutal.