Generally, every game opens up with the villain doing serious damage, deploying huge numbers of minions, and basically making things hell for the players. The game progresses, with the heroes rallying, assembling their ongoing cards and equipment, taking apart the villains' minions and equipment, and finally coming back from the brink to win. Exactly like in the superhero comics the game drew from!
Of course the villains attack first, forcing the heroes to defend themselves and innocent bystanders. It wouldn't be very heroic for the heroes to attack first.
Likewise, when each hero is incapacitated, they gain support powers for whatever heroes are still up — now, how many comic books have you read where the whole super team has been knocked on its ass, only for the last man standing to gain a Heroic Second Wind and pull out a victory?
Absolute Zero's cold attacks that do him fire damage reflect physics — "generating" cold really means taking away heat. Therefore, a system that creates and projects cold must transfer the heat somewhere else. So when Absolute Zero is blasting something with cold, he's creating heat elsewhere — hence Hoarfire, which is a cold and fire attack, and the other attack one-shots where AZ takes fire damage equal to how much cold damage he's putting out. Alternately, he's generating cold attacks by venting the chemicals that keep his own suit cold — so each attack he makes is draining from his own life support.
Legacy is a great leader of a team, and the larger the team the more useful he becomes, considering how many team-wide boosts he can throw out. His base power Galvanize, in particular, is extremely useful as part of a large team. Young Legacy, on the other hand, doesn't have Galvanize, instead using her Atomic Glare as a high damage-dealer. The brilliant bit is... Legacy is an experienced leader who can motivate an entire team. Young Legacy doesn't have that experience, but her high-damage base power is extremely useful for a smaller team that doesn't put out as much firepower as a large team. Their powers encourage the players to use them like they would act in reality: the experienced hero leading a big team while the inexperienced hero assists a smaller team!
Ambuscade doesn't scale up very well against larger teams. He's a serious opponent against three-man teams, a moderate but manageable threat to a four-man team and will likely get smashed by a five-man team. Because... he's a hunter. A smart hunter waits until his prey is vulnerable and easy to bring down, and doesn't blunder into a situation where he's facing overwhelming danger. So of course he's most dangerous when he's up against a small team. One could interpret him fighting a small team as stalking and attacking his prey when they're the most vulnerable, while fighting a large team is him making a mistake and stumbling into a straight fight with a larger group of heroes than he planned.
Legacy's powers are mostly focused on protecting and supporting his allies instead of attacking his opponents directly. In the Freedom Four comic, Legacy went straight for Baron Blade and got his ass kicked thoroughly because he attacked Blade directly and on his own. When he recruits the Wraith, Bunker, and Tachyon, he lets them do the work against Blade while he distracts Blade and takes hits for his team, until the Wraith can ambush and disarm Blade, Bunker can destroy the drill, and Tachyon can punch her way through the Blade Battalion and hack the platform's main computer. This is exactly how Legacy works in-game.
While it's not normally possible, having all three versions of Legacy on the same team would be devastating to villains. Their buffing Ongoings would stack, their defensive ongoings make them immune to the environment and extremely difficult to hurt otherwise and their base powers work in great tandem to support one another. Which, of course, makes sense — that kind of family dynamic nearly always results in amazing feats when it appears in comics. Having three generations of a superheroic line — one of them back from the dead — all in one battle should result in wholesale asskicking and name-taking.
The three most prevalent card types — "Ongoing", "One-Shot", and "Limited" — are industry terms for types of comic books.
Expatriette's outfit is a deliberate fashion statement. On her base card, she wears a white collar, with stripes of navy blue and teal for a top. This is a deliberate contrast to her mother, Citizen Dawn's emblem, which is yellow on the bottom, with red and orange alternating stripes. While Dawn's emblem is meant to resemble a sun rising over the horizon, Expatriette's outfit evokes a moon shining down.
Generally-speaking, the villains that scale up most dangerously are actually the sort of villains that would be realistically dealt with by a small team of heroes:
The Dreamer scales up hard based on how many heroes are fighting her, but she is much less dangerous when it's a small team, which more accurately represents how the heroes "fighting" her would be using minimum force to save her.
Gloomweaver is vastly more dangerous based on how many heroes he's fighting, but that's because a small team of investigators would incur a less extreme response from his cultists than a large group.
Warlord Voss sends more mooks after the heroes based on their numbers, because a large force of heroes is an obvious threat that he'd throw a lot of troops against. A smaller force sent to assassinate him, on the other hand, would garner a weaker response....
The Ennead deploy more members based on the number of heroes, which makes sense, as more heroes would draw more members of the group out to fight in the first place.