'nades, bananas, Falco's lasers, Pit's arrows. I really can't think of any other projectiles that could come close to those in terms of usefulness.
The problem with Din's Fire is that it can't be used to stop people from camping against Zelda. It's very easy to dodge, which means once a player figures out about when it's gonna get detonated it loses most of its usefulness - and if you hit 1/5 of Din's Fires against a well-ranged camping opponent that's still not going to help you much (especially if they've got a long ranged projectile - vs Falco or Pit, they can hit you during Din's and still have enough time to dodge the easy-to-predict explosion).
Seriously, Din's isn't even all that an amazing projectile to force approaches due to its endlag - if they're far away they can approach at a moderate pace and close distance, dodging Din's as they get further into its range. Since Zelda's range isn't so great and she can't really approach, you have to play it smart (though a close-range Din's is rewarding and can land kills from fair/uair/bair/offstage dair (though LK is usually better at those stage places)).
It's not trash, but it's by no means one of the game's top projectiles.
Edit: Falco has been one of my hardest Zelda matchups, I usually switch to Sheik against him. I'm not great, but seriously - her large size, not-so-great fall speed and lack of ranged moves (seriously, lasers lol hard at Din's due to sending it into autopilot) really show through in that matchup.