Everything in MGS1, 2 and 3 were explainable in one way or another. They feel grounded in a "hyper reality" kind of way if you will. MGS2 in particular, gets away with anything since the game was dipping in and out of reality, but at the end of the day, they're games and you're supposed to have fun with the medium.
MGSV on the other hand, was so serious in tone that it makes the unexplained crap glaringly stick out. How exactly did Volgin come back to life? How come Psycho Mantis was so powerful, yet he never raised beyond an average-ranked FOXHOUND? Why did the Patriots let him do what he wants all his life whist they had to control Solid Snake at everything he does? Too many questions, barely any reasonable answers.