They're minor gameplay differences. But also use the same exact body build. They're basically upgraded costumes, but also can have even more gameplay differences if it's easy to do. Chrom is able to use a variation of Ike's Up B, to some degree. Though we don't know how easy it it was. The speculation is that he could only use that move because Ike had the same body build as Chrom/Roy. In Ken's case, they put a bit more work into him.
It also implies that they can still have bigger moveset differences. Ken threw a wrench into our thoughts. Having same body type is the most essential for sure.
We also don't see odd ones like Black Knight, so stuff like armor that could be detached from the models when it came to the actual moveset(so are just extra aesthetics, if you will) might not be possible. as shown by how hard it was for Meta Ridley to work. That extra armor is still extra models, which while they don't affect hitboxes, they are still difficult to program without issues. This is why Meta Ridley was not selectable in the demo. Keep in mind the game itself still has tons of lag and before the final release, we had a ton of graphical issues, as not everyone was looking presentable. Basically, characters who fit the body type but have full armor are likely difficult to program without constant issues. In fact, I'm betting this is part of why Mewtwo never got his Armor for any alt. The other is TPC not saying yes. It also doesn't help that he was either in a game that was rushed or was already having more important things to work on, like making him from scratch for 4.