The limitations of selenium RC are:

1) Switching between the multiple instances of the same browser is not possible

2) Switching between the multiple instances of the different browsers is not possible

3) Browser navigation, like back and forward button emulations is not possible

4) Limited features in terms of drag and drop of objects

5) To work with Ajax based UI elements there are very limited features are there with Selenium RC

To overcome the above limitations we use selenium web driver or google web driver