I've done professional work with Unity 3D before and in my opinion the engine is a great fit for Project Eternity.
The only significant con of Unity 3D is that it's not particularly resource efficient compared to highly engineered triple A engines. But for a Project Eternity this is a non-issue, Project Eternity has (in principle) graphics that systems 10 years ago could utilize, furthermore it's target platform is only PC, so it doesn't have to contend itself with extremely resource constrained consoles.
The pro's are that Unity 3D is cheap*, has a large body of experienced talent, comes with fully features and easy to use tools that allow designers to very quickly mock up any ideas they want to try out. Of all the game engine's that cater towards the non AAA game development market Unity 3D is by far the most established with (by now) the fewest kinks.
*assuming they don't need to acquire a source code license
Plus easy integration of assets, Unity 3.5 even exports .blend files from Blender properly, as well as (of course) Maya and 3DS Max files. This means for the indie developer just starting out, for a grand total of nothing, you can get a game engine, high end modeling software, and a competent texture program (GIMP, lets say) to get started in the gaming industry. But, if you're a big name company with scores of employees and the latest in asset creation software, you're still covered.