Actually that's the point I think you missed. The Darwin's World books don't attempt to define your campaign for you (they aren't world or campaign books), but they provide a lot more than a random assortment of "neat stuff" to pick and choose from (though you could do that, of course). What they do is to provide the elements of a very well-thought-out theme for you to base a campaign on. The classes, skills, items, feats, etc. all seem to complement each other very nicely, both in mechanics and in overall thematics. That is what was so obviously missing from Gamma World.
And I noticed you have some misgivings about balance. To me that's a red herring. Balance is entirely a product of how the DM runs the game. (I don't think that even the standard DnD classes could be judged as balanced except in the context of a specific game.) What Darwin's World does is provide a set of classes that are synergistic. It's up to the DM to "balance" the game, taking into account both the characters and the players behind them.
And I noticed you have some misgivings about balance. To me that's a red herring. Balance is entirely a product of how the DM runs the game. (I don't think that even the standard DnD classes could be judged as balanced except in the context of a specific game.) What Darwin's World does is provide a set of classes that are synergistic. It's up to the DM to "balance" the game, taking into account both the characters and the players behind them.