A malefic represents hardness and strength. Having strength where strength is required is good. When there is strength where gentility is required is bad. And visa versa having gentility where strength is required is bad. So “malefics” (natural malefics: sun, saturn and mars) are welcome in houses that require strength and toughness (3rd for courage, 6th for dealing with challenges, 10th for dealing with the world at large, 11th for being restricted and controlled about one’s own pleasures).
Benefics are welcome in all the other houses. Benefics in 3, 6, 10, and 11 are still benefic, but they indicate misplaced leniency or timidity.
For example I have Jupiter in the 10th house, so I am a bit too optimistic about how the world at large operates, and I sometimes suffer from not protecting myself sufficiently from let-downs and scams.