It might depend on the level he's playing at. Because of it's horrible rampup times, demo will really suffer if by dungeons he means normal, heroic or the lower M+ keystones, because the "pump and dump" classes can put out so much initial burst then everything will just die before demo can hit the ground and get running.
The devs themselves acknowledged this, in that demo is like an old rogue spec, that by the time you've built your resources and gotten everything running, whatever you're trying to do is already over. People think of affliction as the ramp spec, but demo is even worse, the demons are nothin gmore than animated dots, except they have long cast times and you have to buff them with a relatively slow spell on top.
It's probably the same in PVP, demo has to build up it;s dps whereas there are plenty of classes with insanely high initial burst
I get the same problem with affliction - but only in the daily heroic (which I dont; both with now I;ve got enough AP). By the time you've got your dots out and running everything, including the bosses, will be dead if you got a decent geared [insert any burst damage class here] in the group.
Affliction has been pretty horrible for normal/heroic dungeons pretty much since launch, except for AOE spamming. Not even that now really, since SOC has ridiculously long cast-times and they've buffed a lot of other class AOE. It's fine in raids and higher + keys though.
As for the playstyle, I thought demonology was going to be cool. It wasn;t though, it just ended up as something that summons up some imps and a couple of dogs (some "master summoner" then, for heaven's sake, at least give it a fel-lord!) over and over. And then has to buff those demons with Demonic Empowerment. Over and over.
For me it just comes across as toxic to play, it felt slow, clumsy and repetative. and very, very immobile.