I've noticed a very big assumption through this thread... Most everyone assumes that by "adult game" it defaults to visual novel. But that's not always true, there are a lot of adult games that are NOT visual novels. Granted, not all of them are good, but they exist. There are a LOT of adult games made in unity, for example, House Party is probably one of the more well known Unity adult games. You can have an adult RPG, an adult adventure game, hell, pretty much any game type can be an adult game if the content is spicy enough. But yeah, for Visual Novels, I'd say Renpy if it is an indie game. For anything non-static, though, Ren'Py falls behind fast. And an action adventure style game is far better suited to something like Unreal or Unity, as is a 3d RPG style game, as is a shooter style, etc.
What it boils down to, really, is what TYPE of adult game you are making. If it's a VN, then yeah, my vote goes to Ren'Py. But the type of adult game really determines which engine/platform is best.