I'm building a custom editor in Unreal, and the barrier to entry is quite high. I've already added ten new C++ classes, only to make a rectangle with a name on it appear in a generic graph editor. I can move the rectangle around, but that broke my serialization until I realized I needed not one but two different classes to represent just one (type of) node.
Once I get past the learning curve, it's going to be a fairly powerful tool, but there's a long way to go before it works at all.