Con Poor documentation
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Often their examples won't even compile, since they were written for now outdated versions.
Con C++ - oriented development cycle: slow turn-around times
The Unreal Editor is the main place to do stuff (of course), so if someone wants to do a lot of C++ stuff, the compilation and linking turn-around times can be painful. Still they probably are quite fast in comparison to the provided featureset.. Still ,they are far from optimal.
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.