The basic point is that no matter what service you get, you will fail to spot the majority of the traffic that is actually there.
Partly this is because (and this is in the RIS context) so many people don't have or don't use transponders so the ATC has to pass you traffic which is vertically nowhere near you.
Partly this is because most people don't talk to their FIS frequency, or to anybody. Just as well because if they all did, London Info would immediately collapse. Let's say LI says "15 planes in your area". So what? In reality there will be at least 50. You are keeping a lookout anyway. You can't keep a more vigorous lookout, and information which cannot be acted on effectively is useless.
I know this stuff id drummed into our heads during the PPL, but so are so many other myths that have been propagating in the GA rumour mill since WW1.