Din's Fire no longer has any cancels, just somewhat less afterlag. Casting it when the opponent is close to you will render you a free target. It can be clanked like any other projectile, even if the hitbox you use deals only 1 damage. The attack you use doesn't need to be disjointed at all, but you do need to hit the fireball with your hitbox first and not your own hurtbox. Since sword attacks generally don't overlap the character's hurtboxes, snuffing Din's Fire will not work if you overshoot.
As far as Zelda in general, her very design prevents her from being overly technical. She has always been slow and powerful, however she doesn't have the reach of other power characters like Ganon, Bowser, or DK due to her humanoid female figure and is even slower than they are by a significant margin. Match videos involving a winning Zelda don't look very fast or impressive as a result. This gives an illusion that she is "easy to use" and that players "don't have to work very hard" with her to win when this kind of character is tuned up to be competitive. This also gives her a type of Peach syndrome, in which she is effective in lower level play due to people falling into her high power moves without much thought but struggles with her potential in high level play against people familiar with her. At this level of play winning with Zelda is anything but easy with her slow movement and attack speed, which leaves her without much of a great approach or clean offensive game.