I'm on PS4, but seen many, MANY complaints on the PS4 subreddit too.
There is plenty of co-op but it unlocks whilst you go through the game. People seem to expect everything to be unlocked at the start but it's all progressive. You start with a couple of missions that will unlock more as you go on and I'm sure as you play through the story, more co-op opportunities arise. Plus co-op is basically open world exploration and all the collectibles are available as well as the mini objectives to kill messengers/thieves etc.
Unfortunately I'm the only one who has the game by the look of it/share play is a bit broken and I don't fancy going with randoms so the co-op isn't really accessible to me at the minute anyway.
Bit of co-op from the wonderful HatFilms -
Also says in the video, you can only find co-op missions by actually walking past the guy, they don't show up on the map without "unlocking" them so to speak.