Much like most anime that I have gripes with, my main two issues are the story and the characters. Let's begin with the story: in a single word, I would describe it as disjointed. The story is split into several small arcs that feel like they have very little to do with one another; in addition, none of the arcs are particularly interesting (aside from the one that featured the MC from Railgun). The "magic" side of the show feels generic and offers absolutely nothing new to anyone who has ever seen any show like this. Overall, most of the episodes failed to keep my interest.
The characters are my other biggest problem, or more specifically the character motivations. The main character is basically copy-pasted from any other superpower harem anime and he has a martyr complex to the point of attempting to sacrifice himself every chance he gets for a character (usually a girl) that he literally just met. And of course, on the other side of this, every girl he meets ends up inexplicably developing feelings for him because he seems to care so much and because fuck it he's the main character. Almost none of the characters are developed and the majority of them are not interesting.
This series ultimately serves as a far inferior complement to the much more enjoyable and well-written A Certain Scientific Railgun. If I hadn't enjoyed Railgun so much I likely would've dropped this fairly quickly. Although an anime doesn't need to have complex/interesting characters to be enjoyable (see: Hataraku Maou-Sama!), nor does it need to have a particularly original story, not having either ends up only further exposing the flaws of the show. And this show certainly had many flaws exposed.
