Over the course of the series (first and second trilogy), magic portals and teleportation are turning from a common-place and very safe technology into one that is barely usable, dangerous and harmful. Early in the series, small short-range portals can be found all over Avernum, in towns and even bandit lairs. As recently as Avernum 3, the great portal from Upper Avernum to Avernum is described as being unpleasant to stand near, but not overly dangerous.
In Avernum 6, portals are a crap-shoot that are as likely to cause a demonic incursion as get people where they're going. Long-term exposure to a powerful portal - even under the best circumstances - produces effects similar to radiation poisoning; one portal technician is described to have literally liquified. People passing through a portal must be very careful to avoid losing limbs. Overuse of portals can destroy the crystals stabilizing them and lead to disaster.
A plausible explanation for this is that magical knowledge is gradually declining due to constant desctruction and upheaval of the centers of magical learning (eg the Tower of Magi), and the tendency of the more experimentally-inclined wizards to blow themselves up.
(This in turn might be a Fridge Brilliance moment implying that the Empire was actually right to severely restrict magical research and education - not merely to preserve its power, but because magic research is incredibly dangerous.)
