Fireball - benefits from Joint Penalty since it has a fixed AoE ratio. It can also be a link target so you can pull it together with the enemies using Hangman’s Knots
Firewall - benefits from Joint Penalty because each hit it deals is single target. So instead of dealing 17 hits (at Lv15) split on different targets, you’ll deal 17 hits to all the targets assuming they are all linked.
Flame Ground & Fire Pillar - benefits from Joint Penalty because it allows you to hit targets outside their AoE. And for Flame Ground, it also allows you to hit flying enemies (which are unaffected by Flame Ground) by sharing the damage from non-flying enemies.
Hell Breath - lol idk