To some degree, you can actually go apeshit on the missions and still acquire S rank. It's base total time of completion and score ratio on how enemies are eliminated as well as what you used to do it.
You can use D-Walker in that Skull Face mission to speed through the base to the door and achieve S rank even if you alerted the entire base. Your time limit is the only score that can closely get to S rank without the need Ghost rank score multiplier. To imply, it is a bit different than previous MGs but I'm highly sure you can go apeshit in MGSV and still achieve S rank about half those missions. Some of those missions I had to do it to obtain Raiden's suit and platinum the game. You can sneak in MGSV but it isn't a mandatory thing compared to the older ones where you had to do it to obtain even at least a good rank.
You can use D-Walker in that Skull Face mission to speed through the base to the door and achieve S rank even if you alerted the entire base. Your time limit is the only score that can closely get to S rank without the need Ghost rank score multiplier. To imply, it is a bit different than previous MGs but I'm highly sure you can go apeshit in MGSV and still achieve S rank about half those missions. Some of those missions I had to do it to obtain Raiden's suit and platinum the game. You can sneak in MGSV but it isn't a mandatory thing compared to the older ones where you had to do it to obtain even at least a good rank.