It's Mai-Hime's ending that you should be wary of. To set an example, it was such a bad ending that it is now on the otaku lingo. A Mai-Hime like ending refers to an ending that is so bad that you move the series from "best series I've seen recently" to "series I wish I forgot". Compared to it even FMA's seems like the work of a genius. That kind of ending
Mai-Otome is set is a pseudo-alternate universe from Mai-Hime. Although it shares many of the characters, and there is some vague connection in the story the plot it tells is different, apparently set in a vast, vast future.