Archviles are very dangerous, definitely the most dangerous enemy that you see in the game before the Phobos Arena, but I think they work very well in the game as it is--beating Ao100 on M difficulty, for instance, would actually be much easier if Archviles were toned down, because for the last 40 or so levels, on each level your main threat is the Archviles and your highest priority is to locate and kill the Archviles. This makes the game way more strategic than it would otherwise be, and actually much more strategic than almost any other situation in any Roguelike game at all--you have to carefully adapt to your surroundings, the walls and doors and layout of the particular level you are on, in order to stay behind cover and avoid taking fire from monsters which you effectively can't kill because of a nearby Archvile; halfway through Ao100 you have enough equipment, weapons, and traits that you're basically a killing machine, but you can't afford to take sustained fire from a bunch of monsters while you race past them to get to an Archvile behind them. So you have to be careful and sometimes very clever, finding ways to locate and kill the Archviles first, and then cleaning up the remaining monsters (which is usually pretty easy, once the Archviles are gone). I think this is a big part of why Ao100 is such a great challenge.