It might be dangerous, because that whole task is composed of joined user. i.e. users will interact with others.
System might be broken if users was disconnected during task running.
You need to design the logic very carefully.
Edit: I will try to make a global timer plugin of firebase.
I know, therefore i made that each user send information, and how i understand, firebase receives information from user, who sent info the last, user with the most ping. I don't know another ways)
If one user was disconnected, then nothing will happen, but if all users.. It can bring problems