Then again, samádhi (concentration) and viriya (energy) must be in balance. If viriya is more powerful and stronger than samádhi, you cannot concentrate well on the object of meditation. The commentary says: If viriya is stronger or more powerful than samádhi, a meditator's mind will become distracted and restless (uddhacca).
In the beginning of the practice his concentration is usually weak and often wanders. So, he should follow the mind and watch it as it is. If a meditator is enthusiastic for the achievement of insight, he may put too much effort in his practice, thereby causing the mind to become distracted and restless. Effort must be kept in balance with samádhi. To do this, he must reduce his effort, keeping his mind stable and steady; noting whatever arises in his mind and body attentively, but not too energetically. Then he will gradually attain some degree of concentration. Because of this concentration, his effort will become steady and firm, neither too strong nor too lax.
In some cases when a meditator has practiced meditation for two or three weeks, his concentration becomes very deep and strong, the noting mind notes the object by itself, automatically and effortlessly. If, however, insufficient effort is put in, the noting mind will gradually become dull and heavy. Hence, that concentration changes into sloth and torpor or sleepiness. The commentary says: -
If concentration is too strong and effort is too weak, then that concentration changes into sloth and torpor or sleepiness (thina-middha).
So concentration must be kept in balance with effort (viriya). The passive posture of sitting will only make his mind more concentrated on the object and, as less and less effort is required, the mind will become more and more dull. To keep his concentration in balance with effort, he should practice walking meditation longer than sitting. However, only very few meditators experience concentration that exceeds effort. There are also some meditators whose effort exceeds their concentration. Therefore, concentration must be kept in balance with effort, depending on circumstances.
According to the commentary, we can never say that sati is too strong or powerful because it is best for you to be mindful of each and every activity of mind and body from moment to moment. Then mindfulness becomes constant, sustained, uninterrupted and continuous, thereby giving rise to deep concentration. When concentration is deep, insight will unfold naturally and you will be able to realize the mind and body processes (nama and rupa).
So, we can say that mindfulness is never too strong or powerful. from
Vipassana Meditation : Lectures On Insight Meditation by Venerable Chanmyay Sayadaw U Janakabhivamsa