Generally speaking . . . you need to get a read to effectively use the tatsu as a punish for fireballs. The tatsu goes lower body invincible on the seventh frame, so if you see a fireball and activate the tatsu, you’re probably going to get hit. You need to get a feel for your opponents fireball patterns and use it when you think they’re going to throw a fireball, not after they already have, generally speaking. A good time to use it is if you and the opponent are lobbing projectiles back and forth, you can activate it right in the midst of the fray and get a hit, 'cause your opponent will, hypothetically, already be committed to the next projectile.
The opponent’s character is relevant as well, for instance against Guile you’d need a good hard read to nail him, because his projectiles recover so quickly. Or against Gouken you can’t utilize tatsu against his projectiles at all because Gouken’s fireball hurtbox rides higher than other projectiles and will not pass through the invincible lower section of Ryu’s body.
But anyways, I wouldn’t rely on tatsu as a fireball punish too mindlessly, because you will fail and take damage. I would stick to using it as an interruption during a frantic fireball war, when you’re almost certain your opponent is committed to the next projectile. Or do it when your opponent is throwing projectiles with very obvious patterns, or constantly and mindlessly.