I think you have a really interesting perspective as someone coming from games like Vampire the Masquerade. As someone who has run a west marches style hex crawl campaign, I actually found it to be heavy on improvisation and lighter on up front work. I could sketch out many encounters and place them in the world then just flesh them out on the fly. In my mind it's games with more overarching themes and arcs like VtM which I think would be heavy on up front prep and light on improvisation. I think it just goes to show how much of an influence the GM is on the way the game is run, regardless of the style of game they're running.