This seems like a bit of a stretch. Granted, this feature is pretty gimmicky in general though. If your house is on fire to the point that it's likely to spread to neighbors, the embers floating through the air and landing on their roof seems more worrisome.
Here's my 2 cents though.
1) It could lower the water pressure that they need to fight the fire.
By default the Iro starts running sprinklers at 5am. If a whole neighborhood of Iros started their sprinklers at the same time, wouldn't this already be a hazard? What if there happened to be a fire at 5:10am? You're going to need a whole lot of houses already on fire that also have nest protects before a typical neighborhood is going to have a water pressure problem.
2) It could tangle up their hoses, make things slippery, or cause a tripping hazard.
I don't understand. Tangle on what? Pop-up sprinklers?
3) It could surprise firefighters or other first responders when they should be focusing on the fire.
Hah, this made me laugh. So the sprinklers turn on, they get surprised and stare at the sprinklers long enough to forget what they were doing?
4) It could cause unexpected steam, which can be very dangerous to firefighters.
This point is interesting. The way I see it, it's far more likely that the sprinklers would already be on by the time the FD arrived. After all, this whole thing is triggered by smoke in the home.
I have a buddy that works for Calfire and he didn't have any of these concerns. In fact, he said he's actually recommended to some homeowners in the sierra nevada area to turn on sprinklers in their yard when there was a forest fire nearby. The idea being to wet the property in case there are hot embers floating through the air. Ultimately though, he said (and I'm paraphrasing) "Heh, it's not likely to do much of anything unless you had a zone specifically setup to spray your neighbor's roof and drench your whole yard."