e.g., if the dangerous substance in question was a gas mix for the balloon or cooling liquid for the payload, or "Whoops, how did those fruit fly capsules get in there and accidentally drift over California?!"

I'd say the balloons are chosen for a reason. Send surveillance craft over the USA and a huge percentage of the population would see it as an intrusion. Let balloons float over and some of the population will be: just weather balloons, we're wasting missiles; I haven't seen the balloons so they're making this up so China looks bad; what if the wreckage lands on a school, etc. Surveillance and destabilisation, is my guess.