Sometimes it's the little things in the execution of a concept that can make or break a series. Change a plot point here or a character there and the whole thing falls apart. I haven't read either of those series, but I have read so many series that are superficially similar to each other, but the execution can totally break some while others really shine. It makes me wonder how much of that is the author vs. having a good editor/reviewer. Star Wars was famously terrible in its original cut and then was edited just before release to be the mega-hit that we all know. I sometimes wonder how many terrible manga have ended up on an editor's desk that then gets polished to the series we all talk about.