so obviously I wonder, if the invader has equally devistating and cool actions and combos to make the marines' lives a living hell?
The Invader Player builds a customized deck of event cards for each mission. So you choose whether you want your deck to lean aggressive, defensive, or fast etc. (Or you can use the recommended one) The effects of these cards are pretty varied, from bonus movement points to enhanced attacks. Each round you have the opportunity to discard cards from your hand to place Argent Energy tokens on Demons which allows them to use their strongest abilities.
The balancing act is that Event cards in your hand can be used for any demon on the board but they tend to be slightly less impactful, whereas if you power-up a demon they can be devastatingly strong, BUT the Marines know which demon to focus fire on and they might be able to kill it before it has a chance to blow them up.
In addition the Invader can choose which demon army they bring to the table, which alters what comes out of a portal. So in one mission Yellow portals may summon a Revenant while another mission may have it pour out a swarm of Imps. Your event deck can be built to either enhance the strengths of your demon choices or to shore up their weaknesses.
I'm sure there'll be a promotional article up at some point describing it more in depth but that's the broad strokes of it.
I like to think that there's a fair amount of depth to the demon's play, how many cards to keep vs. which demons you want to power up and when, I'm the designer though, so I'm probably quite biased.