Are there any limitations?
Yes there are. If you take a look at the Terragen 4 Product Comparison (http://terragen4productcomparison) page, you can see exactly what they are.
Terragen Free is a fascinating program for doing some hobby art, but isn't suited of course for anything put small resolution personal stills. I used it for literally years and loved it.
I used it originally and loved it so much I bought the Pro version...best money return on a 3D program since Imagine3D