Here's why, going back to the TEB example, if you punch in the TEB5 SID, coming off runway 6, the GNS will navigate you directly to the PNJ NDB, the GNS does not have the ability to fly a 040 heading until 1500' then turn left directly to PNJ, as is required by the SID. The UNS does, same example the UNS in NAV mode will fly a 040 heading till 1500' then turn you directly to PNJ, without any pilot intervention.
Now obviously TEB5 is a ground based SID, not RNAV, but you can see what kind of problems this can lead to as the FAA is implementing more and more RNAV SIDs. So the AFS 400 simply stepped in and said in order to fly RNAV SIDS/STARS an FMS must have proper "path terminators" and the GNS simply doesn't measure up, it has nothing to do with accuracy, simply the ability to fly a procedure that includes heading or courses to altitude, etc.