Level has a HUGE impact on how tough a NPC can be to take down. I played a short scenario with my wife, where she started as a level 1 assasin. I tested it before we ran it, and I had to heavily reduce both amount of npcs and level of the npcs. Also, as in many other rpgs the number of players is of course a thing to think about too.
But I agree, a kind of scale where one can get an indication of how hard an encounter is would be nice.
on a sidenote; when adding npcs to an encounter, the encounter has a level, has anyone found out the funktionality of that one?