Carrara's landscape generator and filter functions are designed to quickly modify a generic landscape. This is the same for Bryce and Vue. Customizing the landscape always involves hand painting. 

Bryce's landscape painting tools are a little nicer to use than Carrara, but Carrara's filters and especially it's Landscape Terrain is superior to Bryce's. For the most part, landscape customization is a close-is-good-enough endevour. 

There are other tools that you can use with Carrara for better landscapes. DCG's Ground Control does not improve Carrara's landscape painting but does allow you import real geological survey map data to create "real" landscapes - from all over the US and some info from other parts of the world. GeoControl is another toolset that seems to yield amazing results - I personally haven't used it because it's PC only at this time and I run a mac shop. 

Vue, only in version 8 - the most recent, has included actual landscape sculpting tools. So, creating the ideal landscape from within the 3D program is not as common as you might think it is. 

Here's the best solution: If you are comfortable working with Photoshop, Photopaint, Gimp or other pixel based paint programs, the painting tools sets are superior to Carrara's. You can simply paint your own gray scale elevation map and then import it into Carrara's landscape tool. If you need to make changes, you can simply edit the original file in the paint program and then update it in Carrara. 

Ahhhh, for the perfect 3D toolset. ;-)