Well you can all pick apart my sweeping turn of phrase but the fact is that most GA traffic does crawl low down, below 2000ft, and this is evident when flying under an RIS - most reported targets that one does spot (which is not a large %) turn out to be well below.
So you are basing your opinion the targets which the radar can see. In my area 80% of the traffic would not have a Xponder and 50% no radio. I therefore suggest that using a RIS as a basis for anything in VMC is so flawed as to be a waste of time.
Rod1