Steam being used for visual novels is definitely a good thing. We’re seeing much more exposure for the titles, with Youtubers and the like jumping on board (though not something I enjoy, it is healthy for the VN market), VNs are becoming more easily available and cheaper, and the use of Steam itself is very convenient, since titles can sit among the rest of my entertainment library that I am already very familiar with. I have not used a platform other than Steam for localised VNs (independent releases aside), but that could be (and is probably likely) due to the lack of exposure from other platforms.
The lack of mature content isn’t something that bothers me too much (and certainly the audience for such titles is very small in the west compared to the audience for all-ages content), and really if you’re looking for something like that then you probably already know where to look.