The open world technique works well because you have so many ways to infiltrate.
That's what I'm liking most so far. Find a good vantage point, tag as many enemies as you can find, then figure out the best way to get in.
Was playing a mission last night and I got a laugh out of myself. While you're scouting, anti-air/gun placements are marked with a square and vehicles are marked with a circle. Well, I didn't really know this yet and the base I was going into had only one vehicle, so only one circle. I mistook that for being my objective I was going after.
When I finally got to the jeep where I thought I was supposed to go, I got really confused. I hopped into the jeep and was sitting there with it running. I was honestly just staring at the TV trying to figure out what I was supposed to do. I knew what the mission objective was but it didn't relate to this at all. All of a sudden, an enemy walked by the passenger door and did not even see me, just kept on walking. I pretty much froze and wasn't sure WTF to do.
My next thought was maybe they don't realize I'm an enemy if I'm in a vehicle? So I started driving and, nope, they know you're an enemy. I quickly flipped the jeep on an embankment and got killed trying to run away. I was fine with that, as I completely fucked it up and wanted to start over. Was still confused on my second play but finally figured it out.