Hoo boy, what a question you posed. A huge topic, and somewhat philosophical to boot. Let’s see what comes out for me.
First off, there’s two kind of difficulty to me - fair and unfair. Unfair is anything you cannot really affect or react to - hits from beyond your reach, hits that come faster than you can physically evade them (with reaction speed and character’s action speed taken into account), hits you cannot see - things like that. Things with no counterplay. Things that seem to break the rules. Some of them (luckily, in this game it’s most or nearly all) may come from buggy behavior, but in some games and instances they’re built in purposefully. Conversely, fair is things you can (sensibly) counter, prevent or react to - through evasion, positioning, patience and so on. Some of what feels unfair is dependent on skill, but not everything, and extreme skill shouldn’t generally be needed to play a game - there are some games where that’s the point, and others where a certain difficulty or difficulties are reserved for the very best, but normally a game should be at an approachable level, and feel fair.
Most of the difficulty in VT2 does feel fair, and the vast majority of what doesn’t seems to be the product of buggy or unintended behavior. The only instances where I can actually think of things feeling unfair by design are the Rasknitt fight (it can have situations where it’s practically impossible to go on, particularly when Deathrattler is down and you’re the last man standing it easily gets to where you can’t get enough reprieve from spell bombardment and adds to revive your allies) and Fortunes of War, and I know (even if it doesn’t feel like it) that both can be affected through sheer skill or earlier choices (but it isn’t always my skills or choices that could affect things). Halescourge’s fight did also feel quite unfair at one point or another, but that was tweaked out.
So, perfect difficulty for me… Is the sort that makes me a better player. It’s something that’s just a bit harder than what I’m used to or comfortable with; difficulty that makes me (or rather, us) struggle through the map and scrape a victory. A match that’s spent completely over 50% health may be nice, but it probably doesn’t feel challenging; one where you and/or your buddies go down once or twice but manage to get yourselves up and through everything feels like an achievement.
In this particular game, most of the difficulty seems to come from things stacking (and overflowing). Elites (including Patrols), hordes, Specials or Bosses aren’t that dangerous by themselves, but once two or more start to come at you things can get hairy. It leads to a bit of an annoyance where 80% of a run can be pretty much just running through, but then a sudden spike makes everything go wrong. And the spikes can come very suddenly; at one point, everyone’s at full health; then an ambush at a nasty moment, a couple of Specials and an accidentally-aggroed Chaos Patrol appears and five seconds later, one character is down, another one separated from others and third caught by a hookrat. Difficulty like that isn’t really the worst there can be, but it’s annoying.
Another thing relevant to both this game’s difficulty and difficulty in general is pacing and intensity. A certain amount of intensity is needed, and a certain amount can be handled, but things can get too intense. It can happen even on normal runs occasionally, but especially hypertwitch on the very low timings get so constantly intense that there’s no room and time to breathe during the run - it’s a constant assault of one thing after another. While getting through that is kind of fun, it also gets pretty tiring fast. Its highly stressful, and I can’t take many of those kinds of runs - while it can still feel quite doable, constantly being on edge starts to take its toll relatively soon. It is (or was; we’ve gotten better at it) my main gripe with the main event on the remade Engines of War - the monks and others come in exactly at the kind of pace that lets you deal with them relatively easily, but leaves hardly any room to breathe in between or to deal with anything else - including the actual mission or reviving teammates.
…And once again I’m left with the feeling that I had something else to say but just can’t catch whatever it was. Oh, well. I’ll add more later if I think of something.