The problem - which is only going to get worse going forward - is that the player base is segregated not just by mission, but by difficulty too.
On the one hand I like that the matchmaking only lasts a minute before giving you AI teammates and throwing you into the game, so you don't ever have to wait too long. But on the other, if you're trying to find actual human players for a higher difficulties, it means you have to keep cancelling and re-doing the matchmaking if it doesn't find anyone, which is a tad annoying.
I think it might have been Voodoo who suggested it back when it came out, but it really could do with a quickmatch feature either for difficulty or missions or both, if you've finished the game and played every mission a few times and just want to get into any match with other human players.