I assume you are talking about the experience of interacting with software applications, tools, devices, and machines because the experience of entertainment objects & systems is very different.
All of the aforementioned objects can be broadly defined as tools that we employ to simplify complex tasks, such as filing taxes, creating a collage of favorite pets, getting from point A to point B, or watching a movie. Thus, their very nature is to be as simple as possible and the goal in improving them is making them easier to use even further.
Usually, tools are simplified through automating and reducing steps in the process of reaching the final goal of the user. And this is precisely where the limit of simplification and the curse of experience design lie.
The secret to a good UX isn't in leaving a challenge to use the tool but in the perception & the availability of manual control to the user. We need to leave the option of manual override in some places and explain the 100% automated steps in other places. Otherwise, users may be left dissatisfied with the final result with no way to change it.
Therefore, a tool is too easy when the user loses the feeling of control over the produced result.