Against which factions is it easier for demons to fight, and against which is it more difficult?

If the opponent is a newbie, you can win without much trouble with any faction.
Although luck can influence the outcome of the battle (for example, morale and luck triggering, as well as special abilities of creatures). I played against a friend (Inferno vs. Inferno). My Nightmares took out all his Devils in the first turn.

Well, actually, one thing leads to another. He's both a newbie and lucky... Although, the simple numerical superiority was the deciding factor. Because after your attack, there should have been a counterattack from the Nightmares and Dogs, which would have eliminated your Cerberuses and probably Imps. But due to the stupid positioning of the troops (Nightmares behind the log, Dogs in the second row), I don't think that happened. So, as an example of luck saving the game, this screenshot doesn't quite fit. And you played for quite a long time, with some unrealistic armies, although it was a mirror match...

About the Machinists. Well, in the add-on, there are more ways to deal with them.