It sadness me but there are very few specialists in conducting research with 'hard to reach audiences' e.g. drug addicts, domestic abusers, children in homes etc. There are two main reasons for this:
(1) Most recruiters are nice 'middle class mums', they tend to recruit from their own network - e.g. people like them, this means that drug addicts, estate kids etc are not generally on their radar + they don't really want to recruit them, talk to them or have them in their own home.
(2) Most researchers are nice 'middle class mums', this means that when taking to 'hard to reach' audiences they are often outside of their comfort zone and therfore find it hard to really connect, probe or understand them. My favourite example is a research director who tried to convince a national bookmakers that they should not speak to DEs despite the fact that this constituted half the clients target audience!?