I have played around with the RTA function of the FMC on the various 'busses I have flown and personally find it very poor for several reasons:
First of all for the system to have any chance of accurately predicting an arrival time it needs the proper route programmed into it and followed. So shortcuts, wx diversions, altitude changes etc. all combine to reduce accuracy. Also the forecast winds need to be accurately inserted for all applicable levels AND they need to be similar to the actual winds...not always the case.
Now whenever any of these parameters change the systems responds by varying the cruise speed. Several hours out it might only be by Mach .01 but the closer you get to the time restricted point I've seen speed fluctuate from just below MMO down to Green Dot (minimum speed). This obviously causes potential separation loss with other traffic if several aircraft are flying hugely varying speeds on the same level/route.