For me, it’s mobile and social media integration. I’m wanting to make a tablet game, and I bought into Epic saying UE4 was mobile ready. So far, half the Android devices I’ve tested on crash at launch, even an empty project just fails to start. When I make a post on AnswerHub, the answer I received was “That device simply isn’t supported”.
My final straw was when I was trying to get Facebook integration. I think it’s fair to say that being able to share high scores and things is pretty important to getting a little match 3 type game noticed (unless some more experienced developers can correct me on this assumption). But there is no documentation that I can find about doing this in UE4. There is something called OnlineSubsystem that is supposed to be an abstract interface into things like Facebook and Steam, but on looking at the source code for the Facebook part of this, there are functions with //TODO, and nothing else in them, it doesn’t look finished.
I’m in the process of learning Unity now, I already miss a lot of things about UE4, and if Epic get mobile to a good place I’ll probably come back to it. But for right now, either you need to be a good enough programmer/developer to implement your own stuff with plugins or just editing the engine source, or you have to wait it out and hope Epic get around to it.