When I first used Flare Blitz on opening night, I was like "AW HELL YA FLARE BLITZ." Charizard immediately became #1 in my stats because of how many KOs I got with him, almost exclusively because of that move.
Then I started to wake up. It's really not perfect. 9 and 4% don't seem to big but if you use it too much it'll accumulate. Also it's really great as a recovery move, but you could easily screw it up. Not only does it do damage to Zard, but it goes really far, and you could just as easily use it to throw yourself off the stage as you could to get back on. If you've used up at least one midair jump (he only gets two extra jumps now instead of 3 and can't glide) and you use Flare Blitz, goodbye. You're dead. Also if you Flare Blitz too close to the edge you will straight up kill yourself! It's sort of like Fox Illusion only even more risky.
The final caution is that if someone shields, you will still take damage but they won't, and the shield doesn't even get any weaker after taking a Flare Blitz, so watch out. And of course Zard will fall down and be vulnerable to an attack after this, so you really have to be careful not to hit someone while they're shielding. If they time it right, they can even stop it with a well-timed aerial. It's probably really hard to do. Also any counter move if used at the right time can punish Zard for trying to Flare Blitz them.
So, awesome move, but not it has to be very well timed. I don't think I will use Charizard too much, but he's definitely one of my favorites.