When the "safe" rating was split into "general" and "sensitive", it became a lot more difficult to work with rating searches. I used to search for rating:s for any safe post and -rating:s for any unsafe post.
Now, if I want to have either questionable and explicit posts or general and sensitive posts, I would have to use two tags. As a free user, I can't use more than two tags, and even for premium users, I would assume it limits the ways they can use the search feature.
This could be solved if you could have something like rating:>sensitive to search for anything more explicit than s and vice versa with rating:<questionable.