One thing you can do in Modo which is awesome is sculpt particles. You can sculpt directly on clouds. these clouds are not good for actual sky clouds but currently have correct features for smoke and such like. This sculpting also means you can sculpt rock arrangements. You have your rocks on the ground and the brush reacts with them the same as if paint i.e. you can swirl the stones around, increase/decrease size etc with a wacom! This is incredible.
Another test I'll be doing is replicating entire terrains. Modo could definitely handle this and you only need a few replications. I've done this in Terragen with Heightfields. You can just use the same terrain a few times but rotate it around. You can't rell it's the same terrain unless you get really high. In Modo it's definitely possible to create a great look of distance. That part is solved. The only thing areas where Terragen beats Modo is really vaste scenes where you are very high or into space and clouds. In my opinion most of the rest is better in Modo.