Cost and complexity. If you have to send so many to overwhelm and bypass all the countermeasures, the cost and complexity of make it a much less practical and appealing solution than just doing it the old fashioned way. I can't see how brass, lead, and gunpowder will ever be more expensive than light weight plastics/composites, electronics, sensors, motors, batteries, plus the actual lethal component. Add to that the required time/complexity required to configure and deploy, situational considerations such as weather, sensor viability, terrain/environment factors, etc... and we're back to going back to guys with guns. Could there conceivably be a scenario where this might be the best option? I suppose, but in my estimation it would likely be the option of last resort.
If an enemy force has already made up its mind to kill, I don't see this making it any easier/more reliable/more effective than well-established alternatives.
That depends on the target. The military is quite ready and willing to spend tax dollars on things even if at the end of it there is something cheaper that does the job better.
If an enemy force has already made up its mind to kill, I don't see this making it any easier/more reliable/more effective than well-established alternatives.