Every superhero RPG needs bad guys. Given that they typically come with character creation rules as a core feature though, I'm not sure anyone really "needs" a book full of them. Yet every superhero game line that goes anywhere has at least one, going back to Champions and Villains & Vigilantes. I still have my original "Enemies I" and "Enemies II" and villains from those books still show up in my games from time to time so they can definitely make an impression and help define your personal setting. They are the traditional Monster Manual of Supers RPG's, a pillar of the genre.
Side note: Given that it's not difficult to rewrite motivations - generally easier than writing up a coherent set of new powers in a lot of games - they can also serve as a handy resource for NPC & allied superheroes as well. It's a book of superpowered people - adjust alignments to your taste!
The Viilainomicon, the original bad guys book for ICONS, had around 50 villains. Adversaries has over half again as many and brings a lot of the old ones up to date with the newer edition and looks to be the "core" supervillain resource for the Assembled edition.
I really like the layout - each character is covered in two pages:
- Front page has game stats and an illustration
(fine work by Dan Houser here again ... or maybe that's "still")
- Back page has background, including personality and motivations and possibly relationships with other characters in the ICONS universe
- Enemy Mime - yes the name is a terrible pun but it made me crack up so here it is. He is sort of a green lantern-ish type in a way because he taps into the "mime force" which can do almost anything but with the limitation that he must physically perform something which then manifests in the real world ... invisibly. So his "mime in a box" generates a physical barrier, climbing an invisible rope actually lets him climb, throwing an invisible ting actually throws a ... thing ... you get the implications. There is a -lot- of potential for fun with this villain and the implications of a "mime force" on your comic book universe are ridiculous with potential.
- Tarpit is a great example of an Animated Series Clayface type character
- Zero is the classic Mister Freeze guy-in-a-cold-suit character
- The Troll has been in ICONS for a long time and is a great example of a serious Brick in ICONS
- The Shadow Guild is a shadowy evil organization and agian it's a nice example of how to do that kind of thing in this game
- Skeletron and the Spartan battlesuit go all the way back to the first ICONS adventure and I was happy to see them here