Also, in case of top tier of biological weapon, even a single strike - or a single accident - has potentially unlimited area of effect, up to and including the entire planet.
Remember COVID-19? Whether you believe in it being natural or a lab leak, it is a good model of how a handling mishap with a mediocre bioweapon would look like.
Better bioweapons would potentially be more targeted, and/or have reproductive clocks that disable them after a certain number of generations. But you absolutely run the risk of them evolving away from such restrictions.
Genetic kill timers are production technology that has already been deployed. There are genetically engineered mosquitoes for example that become unviable after a certain number of generations. The idea being that you mix them into the population, they cross breed and spread their genes, then 10 or 50 generations later, they suddenly are infertile en masse and the whole species dies out.
Remember COVID-19? Whether you believe in it being natural or a lab leak, it is a good model of how a handling mishap with a mediocre bioweapon would look like.