^^^^Have they specifically stated that Borg ships will block someone else's transport beam if there is something specific being transported. I have never heard this. For that matter, phaser weapons on overload could cause massive damage inside a borg ship or installation. If they did block other transport beams because of destructive materials being transported, they wouldn't allow phasers or other overload capable power sources on board either. But boarding parties do get on board with such destructive weapons.
Given what's been shown on Trek over the years, the Borg would not survive as a space-faring civilization for very long....there's too many life forms who would die for the cause and blow em' up.