I'm sure it works for a lot of people, but for me it's kind of ruined pacing in games. Either you end up overpowered for the main game by doing too much side content, or you end up finishing the main game too early and end up unable to experience the side content in the way it was meant to be enjoyed. Side content tends to be more fun if it is interspersed with the main game, so that there is a steady progression for both, and so that any time-gated side stuff doesn't pile up and end up feeling tedious. But when it's left to the player to manage that balance, they are unlikely to hit a sweet spot. At least, I usually fail to do so. Can't plan an optimal route through the game without reading through a bunch of spoiler-packed guides ahead of time, which destroys organic exploration and feels like work anyway.