This is mainly due to the very good AI. If enemies hear a suspicious noise, they search the surroundings; if they discover us, they immediately approach reinforcement. It's very important whether we venture by day or by night. In the darkness, the guards are felt to be more attentive, but cannot discover us so easily. During the day we have a better overview, but so do our opponents.
Later on, armored units will arrive that require special tactics with heavy armor and shields. In a few places we were annoyed during the test about enemies who could see us from a hundred meters in a crouched position, but The Phantom Pain never becomes unfair.
This is especially due to the reflex mode, which slows the game down for a few seconds when we are discovered. If we manage to eliminate the attentive villain during this time, we'll escape the alarm siren and an assaulting mob of enemies. For all the creepers, there's also the optional Chicken Hood, which we can get put on either right after a mission restart or after a few deaths.
The enemies won't detect us for a few minutes and we can easily pass tricky spots. But experienced players won't need the hood, the level of difficulty is very balanced, during the test only one mission with a convoy of vehicles was very difficult and sometimes almost frustrating.