a nightmare for anything else
I dont really think that an engine can supply valuable tools for the "gameplay"

One again it depends on what you are programmer or artist and what you can learn or not.
Some 3D artists use UT3 to make some FPS games, evene some tower defense mixed with FPS.
And if you want ot go beyond FPS , yes you'll have to find a programmer or you'll have to find and learn what you can do or not (visual programming limits, programming complicated or not ?)

All depends on you and your ability or directives (no programming, no 3D art making) ?
And what is your project type.

I just can say good luck to indie people wantign to make Crysis , GTA 5 or Skyrim