Well the difference is MGS5 is garbage and this isn't.
It's not a great comparison. FFXV's open world kind of works. There's some strange problem with the fact that they really should have some urgency in the fact that it's probably not a good thing they are just bumming around the world. That can easily be brushed aside though and then it's not a big deal that the story is lacking in the first half. The first half is a road trip and like a lot of road trips you kind of bum around till you get there. The first part really facilitates that. You can bum around doing side quests, hunts, just running around with your friends, and have little story beats that make sense and your way to your main destination. While yes it could use a few more scenes that are actually not involving the party. Whats Luna up to, whats Ravus doing, whats going on with the empire? Cutscsnes answering those could have been sprinkled through out the 1st half. Still, the open world does not really hurt the story, focus, or tone of the 1st half. Especially since the 1st half is just an adventure with a dude and his friends. It really would have worked if the 2nd half was better because here you would have seen Noctis in his most lazyness, his most imature, and unfocused. Which is kind of the point. The 1st half is all about him not being really focused and kind of taking things for granted.
It's just the 2nd half doesn't build on that as well and it really has nothing to do with it being open world. I think the open world did it's job pretty well. The 2nd half story wise is more focused. Noctis is starting to move past the lazy prince thing and the stakes are being raised. The 2nd half becomes more focused even in game structure. It's just there's not enough story and design to really sell that.
MGSV's problem is that the open world really brings to light how lazy the whole game feels. It's open world, but the world itself is boring and doesn't seem to give much reason to why it's there. The story and characters are just kind of there.