I know most work is done in external 3D modeling packages, but I think with a few extra tools and tweaks the built-in editing tools can provide a decent modeler.
Such as Chamfer, Subdivision, Bezier Curve Dine Drawing, Improved Extrusion, Perform Line Cuts/Splits in place where cursor is located directly on edge by shortcut key, easy line/edge copying and pasting, insert points on selected edges, line division, a Drag-able Degree Based Lathe Tool, and more…
If you need help on how to implement any of these features I have some ideas how too.
I don’t think that’s really necessary, if you’re using BSP at all then it would be simple shapes, using BSP isn’t very efficient in the first place and if you need anything more complex then it’s much better to use a dedicated 3D modeling program and make a static mesh.
Hi KnightTechDev,
Darthviper107 makes some good points about using BSPs. They are mainly used for rapid prototyping and blocking out your level that is later replaced with meshes. However, this does not mean you are not heard! There has been many requests from the community on the subject of having a more refined tool set for UE4. You can see that it is on our UE4 Roadmap via the Trello board. It’s listed under the “Tools” section as “Geometry 2.0.”
While it is listed as backlog/Wishlist right now, a lot of our development is focusing on more prevalent matters such as UMG and Sequencer. Unfortunately there is not time line of when this will be researched or addressed. Keep a check on UnrealEngine.com or the Announcements section of the forums for any updates! 
Thank you!
Tim
The request is valid as the geometry tools are also used for creating volumes so anything that can be done to speed the shaping of brushes is welcome. As Tim said, though, it’s on the road map … it just needs to bubble to the top of the priority list.
It is certainly a valid request. There is a lot of the user base who have not used 3d modeling software or who are just getting started. It can take a while to get comfortable and learn how to 3d model. Having a base set of tools, certainly not to the point of replacing an external 3d modeling software, would be good for new users that want to create something and get their designs and ideas going right away!
There will always be limitations when using BSPs simply because the editor is not meant to be a modeling software. But having some base features that do not hinder a user to create some base geometry to block out their levels would be appreciated for sure.
Tim
It can be used that way, I find though that inexperienced people are using it like they would use a 3D program, in which case it encourages poor workflows
(Similar discussion in Feedback thread
Yes! Vote for Geometry tools 2.0 here! 
For those of us who are primarily concerned with making quality(=many BSP details) maps for competitive play, and less concerned about graphics, being able to make the entire map in the editor, minus custom textures, is the ideal. And this means better BSP / internal modeling tools, like Trenchbroom, Cube 2 and SketchUp have.
Some examples of relatively simple details not possible(or extremely tedious) with Unreal BSP tools:
