Anything which might cause a pilot NOT to keep a good lookout ie through the window is in my opinion dangerous. Total reliance on ADS-B/TCAS prompts pilots not to keep such an 'eyeball' system in operation. Use to ASSIST visual lookout is OK, but where do you draw the line?
When providing FIS to identified traffic, I often give the pilot a clue as to where to 'scan' the sky for a confliction; it's not quite the same a RIS, but it ASSISTs the pilot to fly safely, especially in congested airspace, and the pilot doesn't rely entirely on me to spot conflictions; I'm like an extra pair of eyes in the aircraft.