The sixth house is an apoklima, cadent, and its Hellenistic name is Kake Tyche, Bad Fortune. It carries both of the classical disadvantages at once: it declines from the angle before it, and counting by whole signs it stands at no aspect to the rising sign, so it is in aversion and cannot see the Horoskopos.
The four places in aversion are the second, sixth, eighth and twelfth, and they are the four the tradition reads as difficult. Of these the sixth and the twelfth are also cadent, which is why they are read as the hardest places in the wheel rather than merely as unfortunate ones. The judgement is not arbitrary. It follows from a geometry that can be checked by counting signs.
On the cusp, whole-sign takes the sixth sign from the ascendant; Equal measures 150 degrees along the ecliptic; and the quadrant systems place it as the second of the two intermediate divisions between the lower meridian and the descendant, by six different rules.
There is a further hazard here at high latitude. Placidus and Koch are both built on the time a degree takes to rise, and above the polar circles some degrees never rise at all, so both systems are simply undefined there and produce no cusps. Campanus and Regiomontanus remain defined at any latitude, as do whole-sign and equal. A sixth house reported from a chart cast near the Arctic Circle should be checked for this before it is read.