From what i could see in sandbox games, they are actually more limited than the usual VN story. Most of them have a fairly static story, and your only input is the order of the events (that is if there is no grind involved, in which case it's even worse). This means the characters' reaction & progression is pretty much the same and you have no way to influence it. Classical VNs (in terms of structure; i don't mean to say that Radiant is your run of the mill VN) like Radiant offers the option to influence the way the story progresses, which has much more impact than having to repeat the same scene 10 times, or having to go through 10 rooms at 3 different times of day at 7 different days of the week.
Hopefully this doesn't go the sandbox route (like i said, nothing so far points to it going that way). One of the worst examples of VNs turned sandbox is Indecent Desires, and to this day, whenever i check that thread, all i see people complaining because of the switch from VN to sandbox.
So, let's just see how the next update goes. After playing through it once, and reading the game's writer's posts through this thread, i can only be hopeful regarding it