It has a steep learning curve. It is not optimized for novices. In my opinion, it does exactly what it should do internally, while I consider other tools to be deficient in some respects.

In the common case, undo is provided by git commit --amend. For anything more complicated, I don't think that there is an explanation or GUI which could be considered "intuitive". I've used gitk and git-gui, and a variety of other visual interfaces before Reading the Figurative Manual, and while I'll be the first to cry my own ignorance, I have not found any of them to give much information about even the available options. What is cherry-picking? What about reverse cherry-picking? When would you want to use either?

It's not impossible to design a user interface which would translate all of git's graph manipulations into a simple visual system. However, its current textual interface is extremely flexible, and it exposes a very useful scripting interface. It is a tool that rewards knowledge and experience, like emacs or vi(m). Which is not to say that it's necessarily a good value proposition to you, but it is (imo) a good time investment. One of these days I need to invest in vim too :(