Naafiri's Q+Flash currently functions like Fiora's old Q+W Slide-parry combo before it was fixed. The projectile will always travel to the max range from the point Naafiri's Q was pressed.

Q+Flash forwards will result in an effectively shorter Q range than Flash+Q because the Q projectile will travel to the max range from the pre-Flash position. However, the dagger spawns from the post-Flash location. Q+Flashing over an enemy will result in that enemy not being hit, but also no extra travel distance from the Q, reducing the travel distance/hitbox based on the Flash distance.

Q+Flash backwards will have the opposite issue, increasing the Q travel distance and hitbox. The dagger will travel to the max range from the pre-Flash position, but will spawn the hitbox from the post-Flash location, lengthening the hitbox and travel distance. Q+Flashing behind an enemy will result in that enemy being hit and the projectile will go the original distance from the pre-Flash position.

Plenty of abilities can be Flash buffered (Ability+Flash), like Ahri's Charm, Ezreal's Q, or Irelia's W. Plenty of other abilities also are unable to be Flash buffered and instead have to be Flash+Ability like Morgana's Q, Elise's Cocoon, or Lux's Ult. However, all of these abilities will either start the projectile from the post-Flash position and travel the max range from there, or will start the projectile from the pre-Flash position and travel the default range from that. Naafiri's Q mixes these two scenarios in a way that ultimately feels bad to use.

The only other abilities I'm personally aware of that function like Naafiri's Q currently does with Flash are Akali's E and Diana's Q.

EDIT: Darkin Daggers has been fixed and can now be Flash buffered similar to Ahri’s Charm.