It's not that I don't believe you, but when numbers are involved, asking for data to back up a statement seems like a legitimate demand. Of course, since you 'don't care', you're free to not give numbers, but that makes your statement difficult to take into consideration.
Anyhow, I searched myself and Warui Tatsura apparently did have a viewer share of around 10% (9.4% to be precise :
http://wiki.d-addicts.com/Warui_Yatsura_(2007). However, it aired a 9 pm (a prime-time slot) almost 3 hours earlier than the Detective Conan drama series, meaning that it's perfecty normal that its ratings were higher. It is indeed an example of a drama with mature content getting good ratings, but not an example of a drama airing in a late-night slot
and getting ratings of around 10%.
It's certainly possible that the actors' roles and the expectancies of good viewership influence the choice of a time-slot, but so does the content of a drama and the target viewers.