1) It took me longer to create blueprints than it would have taken to write the equivalent code. I kept finding myself thinking "I could do this in 5 seconds with a couple of lines of code"
2) The blueprints quickly became unwieldy. A tangle of boxes and wires that I couldn't decipher. I find code easier to read, as a rule.
3) I didn't find it any simpler than writing code. A Blueprint is basically a parse tree. It maps pretty much 1:1 to code; it's just a visual representation of code.
I'd argue that 90% of it is just that they work out of the box. You download the engine bundle and you can start doing blueprints. Being able to write normal code requires you to compile UE from source, which is a non-trivial thing for any large C++ project.
I'm pretty sure if they embedded Lua or Lisp directly into the engine, and provided a half-decent editing environment in-editor, that it would meet with success just as well.