The problem is that a map editor is basically a gimped level designer, usually with gimped scripting.
Once you go there, you realize that instead of using some 3D software, and then importing that to a engine, there is possible to make a in engine tool, that can make maps. The problems you then face are the fact material mapping and your material engine may be shit. Really shit. To the point where imported photoshop image files requires massive hacking to get mapped properly. Don't forget that UV mapping for large objects are its own special kind of painful.
Of course, what is the next problem? The editor for objects. Being able to create a level, does not equal you can create or make shapes. For something as complex as it is, i never found out how to make spheres, statues or anything cool in Cube. Once you can do that in engine, in a convenient way, you can go full retarded on making good levels. Mostly due being able to do it fast.
Next? Well, animations. Rigging and animating has always been the most painful of 3D editing. You need it if you want to make complex object interactions, or want a way to script them to work properly with the physic engine. You can give up on rigging something as complex as a human, but having hanging anchored platforms/objects without tons of weird scripting would be a plus.
If scripting integrates well, you can start rigging in armor pieces, hitboxes, and more. Without needing to pray that some half assed importer works properly.
Then the last issue is usually that your editor is gimped in scripting. Or your script language is weird or limited. Or the ability to interact with making map/level is gimped, so there is lots of busy work to get a lot of scripting working.
If you at some point just fire up nano/text edit or vi, just to make edits to scripts, the in game mapper tools are shit.
This is of course only touching the tip of the iceberg. To make a 3D game, you still need a way to write and edit shaders. You need a way to gauge performance. You need good importers/exporters for the things you can't do in engine. You need all interfaces and controls to be good and sane.
You need documentation to be good, accessible, and you need to create a community.
What most FLOSS engines fail at is that they want to be something like Quake or Doom, something you use a external tool to create cool stuff for PvP shooting. They succed at their purpose, but they fail to be game creation tools.
