First off, I'm willing and wanting to be proven wrong. If you saw a rule I missed, point it out.
Second, this will contain some minor spoilers as to content in the expansion.
My opinion is based on having played and owned second edition for a long time, 20+ playthroughs, and now owning third edition and playing a handful of times (probably 5 total).
Going into this expansion, I was excited to have the opportunity to have a party and work cooperatively. In Runebound 2nd edition, you could have allies fight along side your hero, I was looking forward to the possibility of having heroes enter into battle together. I was excited to see new tokens for the monsters to compensate for additional heroes joining together to battle the monsters. Spoiler, this doesn't happen.
There are a lot of questions that I have now that I've had the chance to play cooperatively, and honestly, a feeling of really being let down. The party aspect really boils down to, you move together, and if you happen to have the right party skills that you have unlocked, you get to do some things together. However, other heroes don't engage in the battle as a party (not really, but spoilers), each battle is done as a one-on-one battle. HUGE letdown. From what I could read in the rules, if one player is active, but the other party member wants to shop... can't do it. If there is an adventure card that one party member wants to complete, has to do it on their turn. Really the only thing that happens as a party, is that you move together. The rules even restrict trading between heroes, apparently they are willing to walk together, but if I realize that I have a ton of gold and great gear, but my party member could benefit greatly from having it, too bad, so sad, they have to trade me an equal value of gold and/or items. Wha?!? IN your generosity, you can give the one gold per turn... but even that wasn't laid out in the rules very clearly so I'm not really sure if I can give you one gold on my turn, and then give you one gold on your turn... In other words, there is no group battling, except for minimal skill based benefits, only if you've spent your XP to unlock those skills instead of individual ones, there is no group sharing of items, and there are no group actions, except for movement.
Next, the new enemy sheets and tokens. This made the game go much more smoothly, however, there were times where the timing of actions for the enemy meant that the enemy was busy doing fiddly things, instead of getting in a few more points of damage, and then the hero killed the monster before those actions resolved. Any player would have dealt the 2-3 points of damage knowing his monster was going to die, but the sheet makes the monster fiddle with doubling and flipping rather than just dealing damage before he dies. Minor quibble, and we can fix that in house, but still... odd. The big issue is the Savage class. Holy crap! The other classes are pretty reliable with putting out a few damage a combat round, but the savage class is INCREDIBLY destructive. Reliably they can deal around 5 damage a round, and one round, mind you still in the first age, I was dealt 6 or 7 points of damage...? 4 of those were unblockable. When you have a hero with 8 hit points, and they are dealt half their health as unblockable, that's just brutal. I think this class needs to be tuned, IMO. No other classes came close to the destructive force of the Savage class, to the point where likely in the future, if I face a brutal class, I won't bother, I'll flee. It turned into using wings to try to flip the massive damage tokens just to potentially survive another round, not potentially kill the monster, just survive. That's not a good feeling where you look at the tokens and decide to just flip an opponents token because otherwise you are guaranteed to die.
The last point, and likely a spoiler, is the Locust Swarm mission. Overall, it felt great. We REALLY thought we had no chance to survive until we pulled out a win. Basically what this mission boils down to without spoiling too much is there is no main boss, but a few swarms that spread out, based on the number of players, destroying civilized spaces. Different things cause the swarms to grow, and based on their size they target different spaces on the board and move autonomously. The first issue arose when we looked at the mission sheet, and there was no indication on how many units are supposed to start in each swarm. They need 1 unit to target any location, but the mission never mentioned that you should seed the swarm with any units. There is one rule that states if the swarm can't target any locations, you instead of moving, add a unit to that swarm. We took that to mean they start with no units, and after the first round add a unit. However, that ended up (if true) REALLY being a pain in the butt through the rest of the campaign. Several times when we were fighting the swarm (based on the rules I won't spoil) the fight ended up being meaningless because even if we killed off the two units on the card, they would freely gain one back at the end of the round, and that round triggered the unit growing. Also, the swarm card never mentioned when it would die officially. Would it be when it's reduced to 0 units or total health removed? We decided that it was based on total health, but a simple mention of how the swarm is defeated would have been really valuable (since there are options on what to do with damage dealt to the swarm instead of just to kill the swarm).
I don't know overall how I feel about this specific mission. It did a great job of leaving me feeling like the world was truly going to end, and we needed to work together to stop that, and we truly felt incredibly outnumbered with no way to win but we worked together, got scrappy, took some calculated losses to keep the enemy from winning, and were able to pull out a win on our first try. But, going into a fight knowing you are going to lose, your hero is going to get defeated, but at least you brought the enemy down a little bit, was super depressing. I took the hits so the other hero could stay fit and build up for the final battle, in the end I was glad it was a cooperative game, because had it not been, the other player was truly the winner, based on my constant sacrifices. But, they were never enjoyable losses.
What I enjoyed about Runebound 2nd edition was that you were fighting a ton of monsters, and those monsters were always leveling you up and giving you better gear and stronger attributes, you were growing your party to deal more damage, it was all about getting better. With this mission, we really felt super pressed for time, so we didn't get a chance to grow our heroes. In the end, I spent all of my trophies on party skills, so my hero didn't improve, and I earned a grand total of 6 gold. Defeating the swarms didn't add value to the hero, only helped slow down the rest of the swarm (at least the swarm cards we drew) which really started the whole "is this really coop?" feeling. I was busy killing swarm, which didn't grant me trophies or gold , and the other party member was completing other missions and turning in good for coins. I killed off half the swarm, but my hero was a zero in the second age and couldn't kill anything, and the other party member was able to finish off the game because he was able to complete the missions that rewarded him with gold and trophies. I never had that satisfaction of building a hero, and even the other player ended up with 1 individual skill and 1 item at the end of the game, not a great haul by any stretch. I don't like that about 3rd edition. 2nd edition gave you the opportunity to overpower your hero, if you chose to, the timeline issue was only if the gems ran out or the other heroes were pressing into the main objective. 3rd edition I have always felt weak at the end of the game. I feel like if Runebound 3rd edition was an rpg, the game ends just as you finish the tutorial. This coop expansion compounded this issue by speeding up the game even further. If you go to the end of age 2, you lose, automatically. It would be surprising if you could keep the other lose conditions at bay until the end of age 2, but still.
I still really like Runebound 3rd edition, and I think the fight system makes this a better game than 2nd edition so I would still give this game a great score and will gladly play whenever there's an opportunity, and I did enjoy the FIRST play-through of the coop option, but I just don't know the staying power of this expansion. It never really felt like a party doing things together except for some fiddly bits (not spoiling, it's there), and there were obvious sacrifices that one party, I feel, will always need to make in order to secure the victory for the team, but do I want to go through that again, or do I want to make another player deal with that feeling of "my hero sucks compared to all of the awesome stuff you have"? Now, considering there are 4 other coop missions to try out... this is not a final opinion. Gotta play them all...
TLDR : the savage class is way more powerful than the others (I didn't say broken), sometimes the new token spending sheets make monsters do dumb stuff, trading is horrible in this expansion, as a party you can't take group actions (unless skills are unlocked, even those are lame), the locust swarm mission has some clarification issues, fun experience but may not have good staying power and makes players sacrifice their hero for the party's success.