Hello Alexandre,
Both UMG and Blueprints are extremely flexible and there is quite a bit of documentation on how to work with them. The most commonly used strategy (one that is used here internally at Epic as well), is to incorporate a mixture of both Blueprints/UMG and C++.
Having your Programmers create Blueprint nodes or functionality that is exposed to Blueprints/UMG that your Artists and Designers can then tune-and-tweak without having to explicitly write code, will allow your Programmers to focus on backend or more intricate systems while allowing Designers/Artists the freedom to tune and tweak without having to write code.
Properties inside Blueprints can be set up to be publicly editable or hidden, you can also organize and structure the Content Browser to allow access to certain files if you wish or setup a Source Control solution.
Check out this blog post about Building Fortnite with Unreal Engine 4, it will also give some insight to how we do things here at Epic.
I hope that this answers some of your questions, please let us know if you have any additional questions.
-W