By himself? No, not really. However, it shown pretty clearly that even the The Initiative has been officially decommissioned and shut down towards the end of S4, they still monitor things and have a presence throughout S5 and into S6.
If Riley had stuck around, it actually would have been a good thing strategy wise for Buffy. Regardless of how useless the guys running The Initiative were, calling in the might of the United States military probably would have helped, especially when Buffy was taking Dawn and just running.