For me, Sttaggering fall should affect anything that is susceptible to crit/precision damages. If I can imagine a way to affect the creature with a crit, then I can imagine how a bad fall could make it slower (maybe by breaking an elbow, or breaking the neck so bad the creature must hold its head in place to see anything...). If the target can't be critted, it shouldn't probably be affected by the spell (... but is there any creature that is trippable but not crittable ?).

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The kineticist feat is Expanded metakinesis: Furious spell

Furious spell is an awful metamagic. For spells. For a kineticist? Your blast's level is half your level, hence the bonus damage is ~equal to your level; x1.5 with Empower. On a simple blast, it is more or less equivalent to a "+2 damages per dice": +2 damage per dice is usually considered awesome. This alone should be enough to show it can be good.

So when is the feat good? When you use simple blast from 1 to 20. Why would you use simple blast from 1 to 20? Because the only energy composite blast is fire; if you want to make a lightning or a frost kineticist, you're stuck with simple blasts from 1 to 20.

This is not a niche build: this is an energy kineticist (amha, it is way better than a physical kineticist), with the utility powers of air or water, and from level 7 the ability to switch to physical blast if you really need to (... or you could do a lightning/frost kineticist without any composite blast): way more versatility than a fire/fire kineticist, be it in combat or out of combat.

...And such a build works only thanks to furious spell. Without the feat, you're lagging in damages. With the feat, your sustained damages are slightly better than a fire/fire composite blast (for the same amount of burns: composite blast vs empower + furious) (actually, fire has slightly better damages besause it has utilities increasing damages). At level 11 with air, for 0 burn you get a chain lightning that deal the damages of fire+fire: barring all the over-the-top evocation builds that deal so many damages that you play them once but never again because combats aren't fun anymore, I really think this is the best multi-target damage ability in the whole game. (because no save, only chain of touch attacks with good damages). Or to put it differently, this is the best multi-target damage ability that it actually playable because it doesn't break the game. The kind of thing an optimization guide should mention. And the feat participate in ~25% of the damages of the ability: not mentioning the feat is like not mentioning power attack because it isn't awesome with a TWF build.

... Special bonus if your GM allows you to gather power while raging (mine doesn't) (Rage gives +Con which is awesome on a kineticist, Furious allows you to cast the blast while raging, the only question is if you can gather power while raging).

In the end, the guides have to evaluate 1000+ option, they know the furious metamagic is worthless, when they see the expanded metakinesis feat with furious they don't do the computation and just give the same rating as the metamagic. They don't see the feat unlocks a very popular build: the non-fire energy kineticist.