The mission design for nearly every single story mission is subpar. It is literally ride horse to place (which involves nothing) while listening to dialogue, keep talking, cutscene, try to do the thing, something goes wrong, shootout, escape. It's incredibly repetitive and it doesn't remotely take advantage of the game's strengths. And so much of it is required in order to experience the full game. You can't fish until the end of chapter two, for example. And considering how much money you get from story missions, you're doing yourself a disservice by not doing them. But by the time a certain mission happens, you'll have more money then you'd ever know what to do with.
The game is at its best when you're out exploring without any goal in mind. But even then, the game has situations that will repeat themselves, making the experience feel less authentic. I have seen multiple people with a deadly snake bite, multiple people in a bear trap, multiple people getting abducted; the world is too big to support its comparatively small amount of dynamic events. And honestly, when those aren't happening, there isn't much going on.
The characters are great, the story is great, the music is great, but actually playing the game to experience those things is a chore for me. For a game with this much shooting, Rockstar could have made it more fun.