I think the most important thing with any engine is to not go in with the attitude that you can just open up the editor and make an amazing game without going through some training first. Proper training is everything. Even what appears to be the most daunting game engine can be tamed with a solid foundation in training.
There are 2 sides to my own experience with an engine like Unity. The first side is one where I thought I could just fire it up and make it do my bidding. It seemed intimidating, I didn't understand how the core concepts worked, and I had no clue how to really make it do anything other than the most rudimentary and seemingly useless tasks. But then at some point I was determined to figure out how to use that beast and I turned to a training site for some good video tutorials on just what makes the engine tick and how to do something useful. I was surprised to find out that after just a few hours of training, I was already past the mystifying core concepts and was able to do things useful. A few more hours of training and I was able to venture forth and start experimenting and learning through trial and error.
In the end it took me several months to fully grasp the engine to the point I could utilize it for my purposes. Did I or do I even now know everything? No. And I don't think I ever will. But that's not to my detriment. It's in my favor because that means the engine is well capable of handling my current and future needs.
I also had a similar experience in CryEngine. Opening the editor and attempting anything was even more daunting than Unity. Yet, when I took the time to watch an hour or so of video tutorials, the mystery and complexity melted away and I was making an FPS demo level with hardly any effort at all.
I've also tried Unreal 4 engine and same thing applied there. Just trying to use it without training was impossible. But an hour or so with a Blueprints tutorial and suddenly I had some skills to make things happen.
Bottom line. It's all about proper training. Start with a foundation in a good training program and you'll quickly be doing amazing things in any engine of your choice.
System Specs: OS - Windows 7 Home Premium 64-bit SP1, CPU - AMD Phenom II X4 945, 3.0Ghz, RAM - 8Gb DDR3, GFX Card - 2048MB NVIDIA GeForce GT 640