It blinds everyone for a bit, it's a huge explosion. The Dryads are going to be just as blind as Mathilde and also they will be on fire, which should give Mathilde a chance to disengage. The dryads could maybe dispel, but this is a pretty potent item, I don't think it's that easy to counter.
Obviously there is some danger dealing with the Dryads, no course of action in a battle is risk free. But I think there are definitely scenarios where the dragonflask helps up against them, it's not a sure thing but it's definitely something that reduces the risk.
I see people keep assuming that Drycha is going to ambush us specifically, and...why?
If Drycha reaches the battlefield unimpeded and joins the fight in the time it takes us to kill the Dryards - which she might not - her likelist target is Boris. If she doesn't attack Boris, she might attack the Ice Witch freezing the lake, or the Ice Witch facing that's blasting the Treeman with Ice Magic, or the Ice Witch that's reaping a harvest from her Dryads. What reason does she have to single out Mathilde out of all the forces she is attacking?