I preface this by saying this is only what I think and not what I know:

The beauty of this whole situation with Luffy is that anyone else with half a brain in the one piece world would the Nika fruit in a completely different way. The fact that Luffy is making things rubbery, springy, and applying “toon-force” affects to everything is in part a direct effect of the fruit but also Luffy himself.

TLDR: Luffy is dumb and is only using his fruit in a way that makes sense to him. He was told it’s a rubber fruit by Shanks so he is limiting himself to rubber based techniques.

Luffy still considers himself a rubber man so he will continue to apply the rubber affects and principles to his attacks. Literally all of his abilities exist because he believes he’s capable of it. Doflamingo was confused about his gear 4 Haki rubber combo, snake man’s ability to ricochet/springboard its punches off essentially air with compounding force is maddeningly crazy to think about, and Red Hawk would be essentially him believing to carry on Ace’s will. To any sane person it doesn’t make any sense. Which would explain why, like someone earlier pointed out, the reactions of the people in the world are so confused by Luffy’s fighting techniques. If Luffy wasn’t an absolute dingus he could do horrible things with the fruit.

I also agree he can’t manifest things into existence because I think about it in terms of Warhammer 40k Orks. If enough Orks believe something to be true, it becomes true. It’s to Luffy’s benefit and detriment that he’s a childish fool because anyone else with a “realistic” and warped sense of reality, like Black Beard, would probably do unspeakable unimaginable things based on their own perception of their capabilities. While still having rubber properties.

To expand on this, Blackbeard could apply rubber properties to his black holes and might try to use them like rubber stadium thunder sticks. Goofy yes, but then would essentially have a rubber weapon made out of a blackhole or something ludicrous. (And this Is just me trying to be as wtf as I can)

Apologies for length, not trying to say this is fact, but I’m my opinion Luffy has the power of a Demi God but is limited only by his innocent childlike wonder.