Surely any aircraft in IMC is receiving a minimum of RAS? And so the question is not relevant - you have to rely on ATC for separation? Please don't tell me people fly in IMC without such a service
A RIS would for me, be risky, but anyway, the service provider should be giving the traffic information.
I was flying in IMC with RIS on Saturday. (The "information" was along the lines of "there's lots of aeroplanes in the sky, most without height information, you should be OK if you stick to your current heading".)
And I was lucky to get the RIS. Over much of East Anglia at the weekend there is no radar service available to GA in class G.