Ap or Altap

Are archdemons really stronger than the older ones (more health: 211 versus 199, more attack and defense)? And they can teleport: if an archdemon moves to the other side of the battlefield, it can summon cave lurkers (level 6 upgrade), and then things get interesting. The regular level 6 upgrade is weak: it might kill someone, but if it casts Meteor Shower once, it will become almost useless.

You probably just haven't figured out how to use archdevils properly. Of course, archdemons have a little more health (12 units for level 7 creatures isn't a very impressive difference), but they have less initiative (10 versus 11 for the older upgrade), and summoning Cave Lords is a very good ability.

As for the lords, they're not just strong because of Meteor; "Ray" is often much more effective. And lords can cast Meteor much more often – either when demons retreat to the Light, or with the help of imps, or if they have two Sar-Issa artifacts (which is not uncommon).