In mindfulness both purposeful equanimity and exertion are necessary:
"...even though mindful equanimity is, in some cases enough to uproot causes of stress, it's not enough in all."
"So he exerts a fabrication against the cause of stress for which dispassion comes from the fabrication of exertion, and develops equanimity with regard to the cause of stress for which dispassion comes from the development of equanimity."---MN 101
"Notice that the Buddha offers no specific guidelines for when equanimity will work in giving rise to dispassion and when the more proactive approach of "exerting a fabrication" is needed. This means that you have to discover from personal experience what works in a particular case and to employ the appropriate corrective."
"As for equanimity, even though the passage from MN 101 contrasts it with the fabrication of exertion, we have to remember that it’s a mental feeling (SN 36:31), one of the five aggregates (khandha), and so it’s a fabricated phenomenon. When used on the path, it differs from the fabrication of exertion only in that it requires much less effort. In fact, one of the important insights leading to release is that even subtle forms of equanimity are no less fabricated than mental states requiring much more energy. This point is borne out by the fact that when equanimity is employed instead of the fabrication of exertion, it’s used with a particular purpose in mind: to abandon unskillful qualities. This means that it has an underlying agenda, the agenda of right effort, and is not totally free from preference."
---"Right Mindfulness", Thanissaro Bikkhu.
Employing equanimity exclusively without exertion will result in imbalance:
MN 19 gives analogies of how both purposeful equanimity and the exertion of fabrication are used in mindfulness according to the cycles of the mind:
Exertion of fabrication:
"Just as in the last month of the Rains, in the autumn season when the crops are ripening, a cowherd would look after his cows: He would tap & poke & check & curb them with a stick on this side & that. Why is that? Because he foresees flogging or imprisonment or a fine or public censure arising from that [if he let his cows wander into the crops]. In the same way I foresaw in unskillful qualities drawbacks, degradation, & defilement, and I foresaw in skillful qualities rewards related to renunciation & promoting cleansing.
"Just as in the last month of the hot season, when all the crops have been gathered into the village, a cowherd would look after his cows: While resting under the shade of a tree or out in the open, he simply keeps himself mindful of 'those cows.' In the same way, I simply kept myself mindful of 'those mental qualities.'