I play Random a lot, and honestly, I usually find Vengeance teams to generally be harder than most of the difficulty 4 villains. Not accounting for Advanced mode (I never play Advanced with random), I find Chairman gives me the most trouble, followed by Vengeance teams. Sure, sometimes it'll be a team of low difficulty villains that can be dealt with, but more often than not I just find team villains to be overwhelming in a way that doesn't make me think "Oh, I should have done this instead", but rather in a "Well, I never really stood a chance" sort of way.
Also, because setup takes so much longer, I almost never play them with the physical version- shuffling 5 to 7 decks is enough as it is, shuffling 8 to 12 is a lot of extra work I don't want to go through just to play. That being said, I am much more willing to play them in the digital version.
That's the reason that I never play a 'solitare' style with Vengeance. Every player is responsible for a hero and a villain, suffling and up-keep and all. Makes thing slightly easier if you divide up the bookkeeping. 
But if I want to play by myself, then the videogame is perfect.
While Unity can be tough to play in Vengence, Stealth Bot can be one of the most powerful defensive cards in that playstyle as there are so many villain cards that do 1 point of damage to multiple targets and Stealth Bot just neutralizes all of it (and can't be destroyed by cards that wipe hero ongoings or equipment).
Like others, I found V-style difficult at first but then easier over time as I learned to prioritize targets. It's amazing how quickly the tide can turn in those games once 1 or 2 villains are eliminated. Also, it's easy in V-Style to lose sight of the forest for the trees, by which I mean that there are times when the best strategy is to ignore all of the little targets that are being a pain in your side and take out the villain whose play area they're in - I've found Sgt. Steel to be easier when I just focus all my damage on him rather than trying to take out all of his agents.