It's certainly misleading to do so without saying you haven't played whatever you're comparing. However, I would argue though that making a judgment based on a general consensus of people's opinions of a game is valid so long as you point out that that is what you're doing

Basically you can say what you like in my opinion as long as you say on what basis you're saying it, any intelligent person should be able to evaluate your point given that information and make a decision personally on how valid they think it is

Well if someone reads reviews and watches gameplay videos, you can't exactly dismiss their opinion.

I've played both, too, and I've only bothered to complete Fire Emblem. I'll probably go back to Persona 4, though, since it seemed like I was just getting to the good stuff (after 6+ hours of play).