In unity the process is more or less the same. U need a plug-in though, that expands the standard Unity terrain tools. Its named 'TerrainComposer'.
In terrainComposer it's possible to populate from a color map, black and white map,… So load in the height map and all vegetation maps the same way as in E-on Vue.
Clustering is possible via fractals.
Ofcourse keep in mind that this is real time, so experiment with the frame rate also. All plants have level of detail (detailed object when viewing close, basic object to even static image from far away).
Some screenshots from the Ename testbed. The terrain is 2x2 km in dimension, and contains more than 15 different plant populations. The weather/lightning is interactive and can be changed at any time.
Created by Dries Nollet