the reason that they often can't provide either RIS or RAS cover is the zone of separation required is greater than that required for FIS.
What utter b
ks
Separation has nothing to do with type of service required - purely as to whether the aircraft are VFR or IFR.
The only caveat with this is with RAS
only, whereby you either get 3nm (horizontal) / 1000ft (vertical) separation between traffic known and being worked by you, which increases to 5nm / 1000ft if the aircraft is known to be working an adjacent agency and the mode 'C' data has been verified. If it's not known to you then it's 5nm / 3000 ft. That's why it's only available to IFR traffic.
If you have an IFR on a RIS you separate it from other IFR's and pass traffic on VFR's. If you have two VFR's on RIS you pass traffic information and that's it - you don't separate them.