Players won’t be able to predict the kind of boss that will appear, or where it will appear – unless it’s a part of a rare “set piece” encounter said player might have seen before, but that’s the exception rather than the rule. Players will have to stay on their toes.
The Underworld is not like a dungeon in a traditional MMORPG. Eternal Crusade's Underworlds are more like a run through a dungeon in Diablo II, Path of Exile, or even like playing through a map in Left 4 Dead. The developers are creating a survival-horror experience; players will be deep in the earth, there will be enemies at every turn, players might be low on health and running out of ammo; but there could be an amazing treasure around the next corner. While the development team doesn’t want the Underworlds to be impossible to navigate, they don’t just want them to be a straight walk in the park either.
Some dungeons will even have PVP elements in them, where enemy players wandering past will be notified that they can enter the dungeon and try to take a shot at you. Also, dungeons won’t exclusively be limited to Tyranids either, Brent Ellison gave some examples of Ork dungeons or Chaos Cultists encounters, so this might be an occasion where we could see NPCs for other factions.
It's also possible for players across different factions to be able to team up together to work on a dungeon, so players may want to join a guild to group up with reliable teammates. It's not a priority for the developers at the moment, but they have noticed that a lot of people have shown an interest on having this option, so it may make its way into the game.
Earning rewards in the Underworld gives players and their team resources that can be spent for an advantage in fighting over territory in the open world. Additionally, rare Relics that may be found in the Underworld could serve as powerful items benefitting a player's faction. The development team is still looking into some other ways to make Underworlds impact the open world as well - triggering a Tyranid invasion, for example, but this is just an idea that hasn't been discussed much. The development team doesn’t want players of the Underworld to feel separate from the conflict! Their actions matter to their faction.