For me it highly depends on the actual complexity and enjoyability of the gameplay - a real sandbox game* requires immense focus on the actual mechanics and the interacting systems which is hard and makes creating and actually finishing a game, which already needs a staggering amount of man-hours for anything enjoyable, at least an order harder. Finding an adult game which actually works as a game is rare, I have yet to a find one that I would call a sandbox.
Making a passable linear game, or at least a game with some more constrains on complexity is easier, making interactive story with or without choices and branching paths is even more so, even if it is still a major accomplishment for a small teams or a one-man studio. While I would like to see an adult sandbox game which is not a modded something, for the most part the most enjoyable erotic games for me were interactive stories with at most light gameplay.
*Not the usual event hunting/banality simulators. One of the most complex sandbox games(sfw, or at least nothing erotic) I know would be cdda, in recent releases it seems to going further from enjoyable and more into realistic where it is not needed, but it is incredibly developed and vast.