I give examples of terrible situations with Quick Rebound.
Blade Masters aka RoflKnife.
Enjoy getting re-looped.
Grapplers also have a long enough grab animation to grab you as soon as you get out of Quick Rebound.
High-leveled and skilled Brawlers could also be able to time the invincibility on Rebound and punish with a standing Mount assuming they have Raging Vulcan and the status effect (because they should have inflicted one if they remotely hit you) would be there.
If you were knocked onto the ground by Gunners, it's usually by bullet spam or BBQ, grenades, or something similar. Bullet spam, you'll just Rebound right back into another round. BBQ, they'll just spam bullets like first time. Grenades are the same.
Priests can Smasher, or simply jump and Phoenix Hammer, which has a decent enough splash range to hit you as soon as you're able to move.
Every SP is useful.
I haven't gotten Backstep, and I regret it.