Reiterating, but this game is fun in co-op. As soon as you get even a single partner in there it opens up so much. Not only in terms of approach, but in the variety of sheer madness that can occur as the squad moves through the map. The game is so hands off in its presentation of most of the missions that you're free to be as ridiculous as you want and oftentimes the factions will respond with escalating violence well beyond whatever you were tying to do. The rebels are especially great at making a mess and/or saving you with random patrols and/or planned distractions. Speaking of factions, the Santa Blanca outposts, Unidad patrols, and rebels all combined with the map scale and terrain layout create a kind of cycle of spontaneous firefights and down time that I enjoy. You can find yourself very quickly going from enjoying the massive vistas, to screwing up a rebel mission and chasing down the vehicle you needing to tag, which ends up going off the road and down a cliff only a few feet from an enemy outpost. Then suddenly a stealth mission becomes a big 3 way shootout on a mountainside road with minimal cover and traffic jammed in both directions. When multiple players are involved in this, each scrambling for a vehicle and drawing enemy fire in different directions, it's hilarious.
One thing I do want to point out though, and it's something a lot of games get wrong, is that Ubisoft missed a chance here for something more with the presentation. They undercut the map design and day/night cycle atmospherics by playing to the goofier side of things in voice acting, writing, radio adverts, overproduced mission briefings, etc. I would really like to see this game done with a harder, crime-thriller style edge to it, like an Elite Squad/Elite Squad 2, like a Sicario. Strip back all the way-too-long dialogue, and play up the danger more. Let the sound effects and music speak. You can see them aiming for the Juarez convoy sequence from Sicario at points (pointing out the hanging bodies on a long drive to a mission, for example), but that vibe should be there all the time. Like unless you're going to fill the radio with tons and tons of dialogue ala Grand Theft Auto, just find a bunch of great music to set the scene instead of risking these weird skits repeating over and over. Same for these random 3 minute conversations that loop from the AI squad mates. Just put that stuff in a cutscene and let the tension speak during gameplay.