I joined just to add my 2 cents worth :)
I agree that an export feature would be good and was looking at FlowScape with that in mind. I use Blender and Unreal 4, but any software that is easy to use and shortens the development pipeline is worth a look. If FlowScape allowed me to export meshes (models) and textures to those programs it would get the thumbs up from me. Blender may do it better and Unreal allows heightmaps to be imported, but in the end its about ease of creation, time involved and the quality of the finished product.
P.S. I should add that Unreal 4 does not use meshes for landscapes (they take up too much memory). It stores heightmap data in textures, which is why you can import a .png or .raw heightmap file into the level. This uses about 1/6th of the memory of a static mesh. I point this out just so we are talking about the same thing when looking at an "export" feature and to draw attention to the fact that an .fbx export feature would not be that useful for the actual landscape (just the models) if you were looking at it from a 3D development tool perspective.
P.P.S You probably guessed, writing this a I think of something (messy). However, another tit-bit. Unreal 5 is due to be released in 2021 and the dev's are making much of a new feature "Nanite" that will allow models to be imported (presumably in .fbx or .obj) from 3D software (e.g. ZBrush) with 100's of millions of polys and with no LOD's, baking or impact on performance. Basically, each poly can be the size of a single pixel!! So who knows, this may change the 3D "landscape" (no pun intended) :) Epic has released a video of an Unreal 5 demo running on PlayStation 5 on its web-site for those interested. It goes into detail about the "Nanite" geometry, as well as the new lighting and physics, of Unreal 5 all of which is great news for for creators of large detailed landscapes.
P.P.P.S Sorry, back again, but this comment is mainly for FlowScape. Unreal "datasmith" plugins let users import a host of files from various 3D programs (3D Max, Sketchup, Revit, CAD, Autodesk, etc). Epic has released these plugins for free - here's the link: https://docs.unrealengine.com/en-US/Engine/Content/Importing/Datasmith/index.html I don't know how this fits (really beyond my expertise), but I have used the Sketchup version of the plugin and it does a pretty good job of importing the entire scene (including meshes, materials and textures) into the Unreal engine. If FlowScape was thinking about export capability this may be worth a peek.