In a drone system like this you're probably better off dropping the powder suppressant directly from very low. But that would require more complex custom fabrication of a delivery system. I think "Rain" just used the Fire Balls because of the simplicity of building a mechanism to release them, and not really because of any actual analysis of efficacy.
In their intended applications they're supposed to be hard-mounted where a fire is expected, or lobbed at a fire like an old-fashioned fire grenade... but the slow activation time just seems to rule that out as a good option much of the time. The only really compelling use case I see for them is mounting inside of electrical and equipment enclosures for automatic fire suppression, but there are other, cheaper products on the market for that use.