Mars can be afflicted -- say it gets a square/opp from Pluto, Uranus, Neptune or Saturn. Because, although it is a "malefic", its status as a personal planet makes it susceptible to the influences of the greater/"collective" energies, with potentially challenging effects on one's sense of masculinity, energy, drive, assertiveness/"approach behaviours", etc.
Saturn is a heavier energy, and the consensus is that although it provides virtues that are integral to survival and even success in life, it's basic nature is acerbic and harsh. Astrologers have historically seen Saturn as something to be mitigated or honed in some way. Even when it shows up in aspect patterns with heavier energies, due to its basic nature as a malefic, it tends to be treated with it's outer counterparts as patterns that represent forces greater than oneself that can undermine the will/stability of someone's life. It is notable that prior to the introduction of the outer planets, Saturn was seen as the final planet in the astrological universe, before one went on to the realm of the fixed stars. It's role as the barrier to which nothing can cross has been well established in astrological history.
The elephant in the room in this discussion is Jupiter -- which can also be afflicted. Now why would that be the case, when it is not a personal energy per se, but "social" on the level of Saturn? I think a good way to conceptualise it is that Jupiter's energy signature is not as harsh as Saturn's, so a pinch of Saturn might make a substantial difference in an astrological outcome, while Jupiter's more temperate nature means that it's ability to felt may not be as acute for the native.
One major trait that characterises malefic energy is it's intemperance. Under the Aristotelian Rubric of humors, Mars was described as "very hot" while Saturn was "very dry". One could say that outers' inability or great difficulty in being seen by the naked eye can symbolize a sort of intemperance -- the need to use technological assistance to see these planets being an analog for the ethereal nature of these energies.