Sandbox/open world in this particular instance. I think my early issues are due to the general emptiness and banality of the open world and scarcity of rewarding content (as noted by others), coupled with the weird fragmentation of mission design.
I say this as someone who is really fickle with open world games, as it's a genre that for me frequently slips up with interesting game design. But I feel coherently integrating your entire structure into the open world is essential if you're going to build one. This is something I liked a lot about Sleeping Dogs and The Witcher 3, how they integrated mission flow and core game systems into the open world they had built.
And I think that's what I'm disliking about MGSV, simply the utilisation of the open world, the lack of justification for having it, and how poorly integrated mission pacing and structure is. It's basically at its best with the bit sized structuring, much like Ground Zeroes, but the empty space in between is pretty fucking boring even from a sandbox perspective. Rigid episodic structure was the wrong choice, in my opinion, if they wanted to really make an open world.