So what's the deal with this on USPC? I joined it and it seems to be only Heroes posting in it? Is it cross faction or do villains have their own?
Villains have their own. If you want to talk between cross factions you need to use Verses chat outside in the open world.
If you enabled the LFG in the settings then you will be getting the one for your faction, theres no cross faction in that. If you added it as a custom chat (as I heard it exists) than you might have the hero problem you mentioned. Maybe ask in-game?
The chat name is VERSES which will go to both factions but you need to be in open world (Metro, Gotham) other then that LFG, Shout, Group, Say is all specific to the given faction only. If you want to communicate to a specific villain you can create your own custom chat and invite that person(s) to it