Whenever there are many planets available in 6th, 8th, and 12th house of a birth chart, many astrologers considers that the whole chart is weak or those planets are negative.

But it is not true and final. We cannot simply say planets in hidden houses are totally malefic or totally weak. Because planetary strength and its auspiciousness is based on the particular planet, house, rashi, planetary power in that house, its nature and karakathva about that house, aspect of Lagnesh and other natural benefic planets.

For a Leo ascendant, Moon in 12th house will not make him mentally weak or give malefic results and losses during Moon Mahadasha because Moon gone to its own house and become powerful. If Exalted Mars from Capricorn aspects Moon in Cancer then it will give boost to benefic nature of Moon as Mars is the yogakaraka for the ascendant.

For the same chart, if Jupiter from Pisces aspects Moon, it is not so benefic even though Jupiter is good generally. Here Jupiter is a somewhat malefic for the ascendant and placed in its own house in 8th. But a powerful Jupiter in 8th and powerful Moon in 12th would make the person more interested towards mystical and magical aspects of life.

As the 8th and 12th lord become powerful, the person may go through serious psychological problems in the path of meditation or psychological analysis.

On the other hand, if Lagnesh or a yogakaraka planet gone to a hidden house, it losses its power generally. So it can't give its beneficial results completely. For example, for a Gemini ascendant, Mercury going to Scorpio (6th house) makes Mercury weak as Scorpio becomes 6th house and an enemy house.