In The Attention Revolution by B.Alan Wallace it is elaborately explained why it is important to first stabilize the mind by one-pointed mediation (Shamata) before moving on to advanced meditations such as Vipashyana or other forms of mindfulness meditations being practiced intensively.
If the „monkey mind“ is not calmed to an extent before, there is no use or sense in being mindful of it.
The „monkey“ will seem like King Kong and throw the unstable observer around.
For unstable, untrained meditators this can be a stressful tiring horror show with absolutely no benefit.