I've said this before in other threads, but the hate for FE and sword characters is less of a big thing by itself and more of a sum of several factors.
- FE currently has 7 representatives, making it the third series with the most characters, the first two being Pokémon (8) and Mario (9).
- FE is the only franchise, besides Pokémon, that always gets a new character every game since its introduction. A lot of people feel this is just Nintendo pushing the series rather than FE "deserving" it, like Pokémon.
- Like Pokémon, the new characters always come from the newest installments regardless of fanbase push. It's why some support bases (particularly the ones for Lyn, Hector and Black Knight) have grown kinda frustrated. Again, it feels like pure marketing push.
- Unlike Pokémon, though, not all of the new characters have interesting new archetypes (for example, we had no actual Fighting-type Pokémon before Lucario, nor any character with the wrestler archetype before Incineroar). Several things FE is known for, particularly its mechanics, are largely ignored. I've seen FE fans even complain about this in the stages that represent the series.
- The fact that the characters chosen tend to be the (usually, from my understanding) Lord-class protagonists and these mostly use swords doesn't help.
- The thing with sword characters is that even if their movesets and normals have some differences, they tend to borrow a lot from one another. Sure, this saves development time, but the point is that there are several things a sword-user could do that haven't been explored yet. I don't think I've ever seen Meta Knight and Cloud get as much hate as the Links or the characters that draw from Marth. Maybe a character with a katana, rapier, dual blades or maybe another character with some extra mechanical layer (like Shulk) would also be less criticized.
Now, I don't know all that much about FE and don't necessarily agree with all of this, but these are some of the reasons I feel make people frustrated with FE in Smash.