> But there is no objective or opposition, not even a story (a little robot tells you to do stuff). It's a sandbox, an engine one could build a game in (and they might someday?)

I've found a very different experience. There seem to be several main story threads to follow, as well as the addition of a few interesting sidequests (I'm in the middle of the one about hacking and reverse engineering one of the sentinels). And that's not even counting the expeditions they come up with from time to time. I particularly enjoyed working through the one that had the Normandy from Mass Effect as the end prize.