I really hate the Opportunity Objectives. Obviously, these are missions that forces you to explore the whole map. The thing is, it became too repetitive for me. I wished they changed every Opportunity Objective rather than sticking to the "scan 5 of these," "tap 4 of these," "disable 6 of these," "track 8 of these," etc.
I really got fraustrated because in 3 levels (cargo ship, battery, seoul 2) I missed "1 of X." I already finished all main objectives but I got stuck in a game of finders keepers for hours. I managed to finish all the objectives with the help of hints from other people/friends.
This is my first run of the single player campaign. I'm doing it at Expert level and trying to make it 100% as much as possible and limited saves. I don't even knockout enemies except for the required ones.