I expected it would be difficult and unfair to match something as dramatic as the backstory that FMA had without doing something very similar again so as long as the story develops into something bigger and multi-layered from these exciting but humble beginnings I won't be disappointed. However, despite the greatly like-able characters, if the story doesn't grow as the show continues and remains this straightforward then I agree it will ultimately be a disappointment in the long run as I expect that with so many main characters that Arakawa can build this into a nicely layered story as it continues, maybe even doing something interesting with Keirou later on.
If it remains like this throughout then you could be right, it could be a let down because by the end I'm expecting something less superficial with atleast a little more depth from the author of FMA, even if it doesn't reach some of the massive highs that reached I do expect from Arakawa a multi-layered story that makes the audience get involved or think with thought put into a lot of aspects of it and some of the darker elements of the first two episodes are certainly capable of creating something absorbing as I've been hooked by the three main characters. I'll give her time though as Jushin Enbu is starting from the very beginning of a story while FMA was built on it's tragic backstory and it has plenty of time to still pull off some interesting plot twists.