Pace
There is an element of risk in all flight at "GA levels".
OCAS, there could be somebody there, and gliders only just show on radar. I have flown close to huge h/a baloons and the Farnborough radar controller saw absolutely zilch. Statistically, the risk is very low - ~ 1 midair a year in the UK and most of them below 1000ft.
ICAS, there could also be somebody there - busting CAS. Let's face it, much GA "navigation" hangs on a shoestring. The risk is just lower but it's still there.
In IMC the risk is very low indeed. No IMC midairs in the UK since at least WW2.
CAT has a very low exposure (just as well, eh?) because they climb at a few thousand fpm and are quickly above the levels at which any stray GA will be found. The exposure from busting GA traffic is mostly on the descent, when on the glideslope.
In VMC, the best protection comes from altitude. At FL100 there is no casual GA, and anybody that high will know what they are doing. I have very very rarely even seen another aircraft on any airways flight, at around my level.
A radar service is worth something but not a lot because a lot of traffic (which I am sure does show up) is unreported.
But you must know all this already.