Well, the issue that
all sandbox (as defined by tags) games have is to be released updates after updates.
Such games worth being played only if, and therefore when, there's content concurrently available in more than one location. Something that is difficult, but not impossible, to do when you release your game by updates. Either you add a worthy enough amount of content by update, and it will only interest a part of the players, or you release content that will interest all players, but then the update will be played really fast.
It's not an unavoidable fate, but it need to radically change your development process and, of course, it lead to more times between two updates.
A game that achieved to do it not too badly, while in the same time relying on a real sandbox mechanism, is
Super Powered. There were too much grinding, but globally it have everything that is needed:
- Content available in most place;
- Place that were not always filled by the same character(s);
- Content that evolve through time;
- A variety of content;
- Most scenes that were renewed, depending on the previous actions.
But the flaws were also present:
- It needed 6 years to reach it final state, and the game is in fact not finished; it just have an acceptable closing scene;
- The game stopped to fell empty only after one and half year of development;
- The lewd content was mostly really limited in size, with many small scenes and only few big ones;
- The delay between two updates was relatively long (for its time);
- There's more than 200 hundred identified subtle bugs.
The rest have been addressed by the previous answers, so I'll stop here.