As I see it the problem with a bioweapon (living ones at least) UV light would kill it before it could do any damage.
And besides. The Chinese released a bioweapon using commercial air travel with Kung Flu. Why bother with a balloon?
It is the Biden cash delivery balloon force. They aren’t exactly learning anything new with a balloon that the Chinese don’t already know from satellite. This is just probing our weakness and response. And likely, I believe, launched from Chinese submarines. No way these showed up floating all the way from China undetected. (Assuming anything we are told is true)
Balloons are often used to sample the air over or downwind of nuclear weapons sites to check for telltale isotopes.