So, after Roy came out, I saw this thread and I've been skimming it for a while. I decided to play Fire Emblem 6 myself and see what side of this I fell on. Im not far into the game at all, So my opinions may change as I delve more into this.
Personality wise, Roy isnt... Incredible? I see what they're going for, he's supposed to be a passionate young man, doing what he thinks is right, even if it isnt always the most logical plan. In Chapter 3 he literally wants to go storm an enemy stronghold with the help of a local thief who knows the lay of the land. Not a terrible strategy, but the whole raid is a bad idea to begin with. STILL, He's just a boy. He's just not experienced.
I think a big part of why people think he's bland is because he has the same vapid expression on his face all the time. You really have to try and pull the expression from the text, because everyone seems pretty wooden otherwise. The game may have benefited from multiple expressions...
In terms of gameplay, he seems fine. I've played FE1 and 3, and Awakening, and even though he doesn't seem as invincible as Chrom or Lucina, he's at least nimble. He dodges all the time, and deals a lot of damage. He still has to be protected though, because unlike Chrom he cant take on entire armies by himself. But he sure can hold his own against tougher units like Armor Knights.
I'm not sure I understand yet why he's considered so bad.