The first trailer was only a development in progress trailer, so they probably meant for it to be a lot different back then. Misleading in hindsight but ah well, impossible to change at this point.
Revealing it too soon was the prime mistake regarding #FE.
The game was far too early in development to the point nothing besides the usage of the SMT and FE franchises in a collaboration project was decided. It was far too soon to make it public.
The way the trailer was presented set up expectations far different from how the project eventually turned out to be.
Would someone watching that teaser ever expect a game about J-pop with some SMT and FE sprinkled in?
This isn't like announcing a Smash Bros. title too early, as the Smash Bros. franchise is very well-defined in regards to its identity. When Iwata announced Brawl and Smash 3DS/Wii U before they began development, people knew what was coming and got exactly that: Smash Bros. titles similar to the N64 iteration and Melee, surprising/unexpected/controversial roster additions notwithstanding.
Again, the problem with #FE isn't the J-pop thematic itself, even though I still defend it's out of place for a game that combines the SMT and FE franchises. It was how it was presented in the first place. First impressions are very important, particularly in marketing.
If Nintendo had just stated they were simply doing a collaborative project with Atlus alongside their subsidiary Intelligent Systems and how they could borrow from the SMT and FE franchises, the reception would have been far less negative as such a statement would have better described the project at that point and would have served (even if only in hindsight) as a more catch-all designation for the project.
The teaser gave more of an impression that it would be a straight-up crossover between Fire Emblem and SMT, which did not happen, thus betraying expectations. It won't surprise me at all if this game bombs in the West, just as it did in Japan.