What you have hit on is the lack of balance. Regardless if they are an adult game or some other vanilla game, it comes down to execution of the game mechanics that's playable, interesting, and has appeal. Add in the "adult game" you have to have story and plot that all stimulates. And there is where I find disappointing in many games I have looked at. If your into watching porn, you will know that you can Google an aspect and you will find it on many pages and sites. Different characters doing the same thing--so if you seen one, you have seen them all (yes rather over simplistic but generally true.)
Now a good adult game needs the script, plot, character development to match the games theme. I am envious at the abilities of game developers' ability to build the game, and the artist ability to create the visual. But there stands the first weakness. Who is really going to spend the time it will take to really build a solid game. Its labor intensive from what I have researched.
But even more difficult is putting an idea together with a game so that the labor involved isn't wasted. It comes down to them making the player caring about what happens next, to unlock the next bit of the game; it comes down to plot; it comes down to character development (which is critical);and it simply comes down to who are behind the game that makes it a hit or flop.
Want a great game? Have a theme that appeals so talent can flesh it out. Someone might be a good dev, someone could be a good artist, while someone can be a good script writer but a weakens in one is a weakens in all when the final product is put out. And that all takes time to produce.