Even with Radar Information, there are many planes (e.g. gliders) which the radar can't see, and many more which have no transponder, or have it switched off, perhaps intentionally.
Well, my radar can certainly see gliders and non-transponders. I don't think there are too many SSR-only units out there, either. Under a radar information service you can expect to be told about both transponding, non-transponding, no mode charlie, whatever (subject to the controller noticing it, and radar coverage of course
). On that subject, if you have mode charlie, PLEASE use it. It makes a huge difference, perhaps most of all to London Military who don't have to avoid you with their Nimrod's and AWACS's up at Flight Level Nosebleed when they can see by your mode charlie that you are down in the weeds.