I liked the first chapter, more than some series that I read from Jump lately. It has mystery and it is quite serious, too. Regarding the art, I have the theory that it changed from the middle to the end to bring a contrast between the nice life that he was living and the desperate situation from now on. So, I believe that it was intentional, otherwise there would be no reason to the art evolve from a page to another.
His power has margin to evolve, it always has, it can seem a great power, but we don't know the level of that demon who he fought, I think it was pretty low. Well, looking forward for the next chapter. I want to know what happened with Tsubasa-chan and the others. There is a huge gap, he simply wake up in the middle of the street. o.o