Most of the "documentation" for code is actually just automatically generated from the source. If you're interested in knowing how things are supposed to work, you must either go to their answers site or pay for UDN.

Often their examples won't even compile, since they were written for now outdated versions.

Making a full rebuild, including engine can take a good 30 minutes. If you plan to use Unreal professionally, you better get some licenses for Incredibuild as well.

Compared to other engines, UE4 seems to perform various actions considerably slower. Actions like starting the engine, opening the editor, opening a project, rebuilding shaders, updating references, calculating lightmaps, saving projects, etc take long enough to get irritating and end up wasting precious development time.

A blank project will build in to a minimum of 200 MB.

While the source code is available, the license is still proprietary for the code content (not just the creative assets such as textures and models). This source code is under a strict proprietary and NDA agreement.