So the worst issue with ghasts right now is their aim. They're definitely aiming half o' block above the player's head, that makes deflecting fireballs literally impossible sometimes, especially combined with the fact that they like to float above you, so fireballs are targeted not even in you, but one block behind you. I think they always aim above the head, but misses occur only if ghast is significantly above the player, distance doesn't matter. If ghast is directly above the player, it shots one block behind the player, if it is some distance from the player, but is not directly above, something like 45° relatively to the player, it aims ~0.5 blocks above the player and misses.
Deflection of fireballs that are aimed to the player by ghast that is at an angle of ~60°-90° to the player is impossible without moving.

To replicate:

1. Get yourself an unfriendly ghast by any means
2. Get yourself below it at an angle of 45°+
3. Make it shoot you
4. Note how fireball consistently goes above player.

Note: it might still hit you with an angle of 45°, but should always miss when right above you (90°).

Video with testings: https://youtu.be/188D9cjzooA