My experience with Sandboxing was... it is a lot, a lot of work, somehow it felt like running a theme park, and I needed to create a town of NPCs, all with interconnected stories that players can catch-up and follow, with "rides" or rather events that happen to spice up the play, instead of just going around playing 20 questions.

What happens though, is that if you tell everyone that it will be a sandbox, they'll go ahead and be very creative, to a point they'll throw a spanner in the works, and you'll see those carefully created NPCs with rich backgrounds, connections and quests die at the hands of mad murder hobos.

(Not always though, experienced players understand this is a theme park).