Well, I've seen a few mentions over the internet of how the girls don't seem to be all that powerful in this movie - example in mind when saying this, and what initially made me think "wow, that was lackluster" was when Hayate uses that attack on the "construction equipment" being controlled by Iris. It does nothing to it at all.
I just wanted to point out,
the idea behind formula is that nanomachines analyze energies and materials, and change the molecular structure to counter them. Iris obviously already did this with the equipment she "possessed" and both her and Kyrie very clearly did their homework on Nanona, Fate, and Hayate, including their abilities.
It makes perfect sense that they would be so ineffective against the machines being controlled directly by Iris, unlike the mechs we see later, summoned by the 3 from the book. To be fair, this 3rd movie does a terrible job pointing out that Iris / Amitie / Kyrie are not just using some kind of "different magic" power, so it's easy to think "man, the girls can go from destroying cities and vaporizing steel and concrete ...to not being able to destroy a bulldozer mech"
Now.... at the risk of spoiling any detail of the next movie for those who have not seen the followup, I'll tag this as spoiler too... though it doesn't actually give away any EVENTS... so it's probably safe. I'd just never forgive myself if I spoiled anything Nanoha for anyone that didn't wanna hear it before they saw it.
Though this is explained in a little detail in Detonation, look at the difference between when Fate or Hayate or Signum attack Iris / Yuri / Kyrie vs. when Nanoha when Amitie shares "Formula" with her...
If anything, the girls are stronger than ever... not to mention everyone else like Yuno (protecting entire islands) and Chrono (uh, yeah, wide area attacks). I had that initial idea too, that they seemed underpowered, but I analyze the hell out of these Nano-movies (and the series... of course) and after doing so... no, they only get progressively more badass.... including the Wolkenritter who unfortunately don't get nearly enough badass roles outside of the series.
at least, until StrikerS. Signum / Vita get pretty hardcore at that point, despite the fact that they now have limited life and power