I've put a good 70 hours into the game and nearing towards the end, but not yet quite close. It's mostly a Shin Megami Tensei game, but with Fire Emblem elements added to it, as well as characters borrowed from Awakening and Shadow Dragon. Once you reach a certain point in the game, you understand why other characters from different Fire Emblem games weren't chosen. Awakening and Shadow Dragon don't share the same timeline, but do exist in the same world and realm (Ylisse being the future Archanea). And so, for story purposes, it was limited down to characters from those two games.
It's understandable why folks are so outraged by the game. It's not what any of us expected, especially since the teaser was highly misleading. BUT... I find Tokyo Mirage Sessions #FE really addicting. Although the J-Pop is a bit cringey, I have to say, this game is very fun. It's a weird and wacky concept, but still pretty enjoyable and unique nonetheless.