Although Evan has a Devil Fruit power that could potentially be world-breaking, there is ample thought put into his training and own mental limitations to keep him from becoming too strong too quickly. It takes him a lot of concentration to control water, which is something he doesn't have a good time with since he's ADD. Also, his misunderstanding of scientific principles and the exceptions of these that water has leads to him botching moves that he should be able to pull off, such as the Master and Crazy Hands which he used early in the series. Furthermore, he's completely a Glass Cannon, and would not be able to survive a majority of the crazy shit the Straw Hats do.
Evan's story is one more of physical growth and his efforts to change events for the better more than personal character development; while he certainly has his moments, he starts out as a character who is pretty comfortable with himself and is fairly adaptable. He is pretty well-rounded mentally and doesn't have any categories he sticks out in as lacking in, except perhaps a solid work ethic. This is shown by him skirting around training with Zoro, among other small moments. Besides that, the majority of the story revolves around trying to change himself so that he can keep pace with the Straw Hats. It also heavily revolves around him trying to change events in ways that will result in an easier time for his friends and less casualties overall, while at the same time keeping them prepared for the trials to come.