Oh boy. I have a lot of different Fire Emblem opinions, but for some reason they're hard to remember right now. So I'll just write a few that come to my head as I'm writing them.
I hate Pegasus knights, with an exception of a few select characters. I just feel like, even though they have SOME good stats, they don't do a lot of damage, and don't add anything to the game. I think that there's definitely much better classes out there then to waste a slot on a unit like that.
I'm not a big fan of Cavaliers, in Echoes or in any game. Hey, Mathilda was great, both stat wise and personality and all, but I didn't use her, and even though I regret it, I still beat Echoes no problem. Zeke was pretty good too, and I actually used him. Cavaliers aren't really a big deal for me, and they don't really change the game in any way. I just bench them halfway, or until I recruit a better unit. There are way too many Cavaliers in Fire Emblem, and most of them are bad units or are overshadowed by much better units.
I have no problem with admitting that I like Faye. Who cares if she's a totally new unit who wasn't in the original Gaiden? I feel like she was needed in the game to add something that was greatly missing from Alm's side. She's nowhere as bad as Camilla and Tharja. Faye's nice to everyone in Echoes, even Celica, and even I thought that they would hate each other. I don't think that her so called "obsession" with Alm gets in the way. She's not the only character to be obsessed with something. Peri is obsessed with killing, Felicia and Jakob were obsessed with their job and the Avatar, but I understand that since that was what they were hired to do. So Faye shouldn't be singled out because of that, and there's more to her than Alm. I just can't believe how much negativity she gets.
Lyn is a horrible Lord. She starts off with decent stats in Strength and Skill, but that's all there is to her. She is a super fragile unit, with low Defense and almost no Resistance. Even though I didn't finish FE7, I'm pretty sure that Eliwood and Hector would have turned much better than her.
I can't stand Azama. For some reason, I just don't like him. I feel like he's a bland character, especially since he got stuck in Birthright, and he takes so much work to actually make him a decent unit. He's annoying, rude, and disrespectful to everyone.
The only royals in Fates that I somewhat cared about were Leo, Takumi, Elise, and Sakura. The other royals were okay, I guess, but it didn't matter to me what they did, their type of character was already set in stone, while Leo, Takumi, Elise and Sakura had a bit more flexibility in which you could see how they grow as a person as the story progressed. Xander and Ryoma are overrated, Hinoka is just boring and a Pegasus Knight, and you already know how I feel about them, Camilla is, well, Camilla. The four royals that I care about turn out to be some of the best units in their respected games, and grow into better units the minute you start training them. Not only that, but I also think that you need these units to have a better shot at beating the game with these units, even though you can solo Birthright with just the Avatar and Ryoma.
Gray is the worst Villager in Echoes. He doesn't really have a true "canon" class set out for him, like Faye, Tobin, and Kliff do. You could through anyone into the Merc line, and they'd turn out pretty good due to the way Echoes is. Gray doesn't really fit for any class besides Merc, which means that he has the least flexibility.
Ike is an overrated unit. I just don't understand why he's so popular. I mean, he's a cool unit, but he definitely doesn't deserve all that attention that should go to other units.
I have so many more opinions, but here were the ones that came to mind. I started writing this almost 1 hour ago lol, so please don't think that I stole anyone's opinions, I didn't mean to make it sound like that. These are just my opinions, please don't get mad at these.